Gear Third is another powerful form that is first hinted at during the climax of the fight between Luffy and Blueno, and first used, but not witnessed, to knock down a steel door in Enies Lobby. It was first fully seen in Luffy's fight against Rob Lucci.
Luffy corks his mouth with his thumb, bites into his thumb joint (making a small opening) and blows very hard into it, which inflates his arm. He is then able to transfer the air through his entire body, requiring the torso to act as an interface for his various limbs. Luffy blows so much air into his body that when they are all focused on a single limb, they become the size equivalent to (or greater than) that of a giant's, if anything resembling Oars' size. Used with his already-strong physiology, this apparently gives his arm more mass and a larger area to attack with far greater strength, but he sacrifices mobility for this strength, as his limbs are now so large that Luffy has trouble moving. This is called Gear Three in the Viz Manga, and Third Gear in the FUNimation sub and dub.
Gear Third is generally used when raw physical strength is needed, the bones are the key difference between this form and his old "Gomu Gomu no Fusen" move, where his balloon form was soft and squishy like a regular balloon, whereas the inflated bones are very hard, similar to an inflated car tire. Unlike the normal Gomu Gomu no Fusen technique, bullets ricochet off of Gear Third instead of bouncing off. The bones being much harder also mean a lot of compression of air takes place when using Gear Third, making Gear Third not just a "for-show" move as Lucci initially thought, but like a giant mace of astounding mass when used in attacks. Because of Luffy's raw strength, he is able to move his gigantic arm with ease.
In between his attacks, Luffy brings the air back to his chest and then shifts it into another body part. The downside is, the lag time in which it takes Luffy to transfer and re-channel the amount of air trapped inside his bones can be very disadvantageous.
Before the timeskip, while in Gear Third, it proved quite difficult for Luffy to move while part of his anatomy has been converted to gigantic proportions, making the Gear inadvisable to use against opponents capable of moving at high speeds (like Lucci) or in an enclosed place as he is leaving himself vulnerable to attack, which is why since his battle against Rob Lucci, Luffy tends to only activate Gear Third against massive and often slow-moving targets whom he likes to assault with his Gear Third moves coming from high above in mid-air, where the said attacks could not be obstructed. To deactivate Gear Third, Luffy leaves the form by exhaling heavily, which can cause him to be propelled backwards.
The deactivation of Gear Third brings about shrinking, which is a negative side-effect for Luffy due to the massive expansion undergone prior to release. The amount of time Luffy spends in his shrunken form is roughly equivalent to the amount of time spent in Gear Third mode. This shrunken form has shown little to no ability to stretch at all, and it has been shown that Luffy loses an incredible amount of strength when he kept hitting Lucci's face to no effect. The best strategy Luffy could use, while waiting for himself to return to normal, is to run and hide from an opponent for the duration of the shrinkage. Another strategy is only using Gear Third for a few seconds for only one move so his shrinkage is minimized. But during the Impel Down Arc, Luffy demonstrated using his Gear Third with great speed. Being fast enough to defeat Jailer Beasts, who possessed Soru-level speed. During the Whitebeard War, this drawback was not shown during every use, perhaps being glossed over by the many different battles taking place all at once; but the anime still includes the drawback.
When Luffy used Gomu Gomu no Gigant Fusen during his encounter with Fleet Admiral Sengoku, Sengoku himself referred to Gear Third's increased size as Gigantification.
After the timeskip, as with Gear Second, Luffy seems to have obtained better skill at utilizing Gear Third. Previously, Gear Third would increase the size of the majority of Luffy's arm, thus reducing the speed of the attack in exchange for a powerful impact. However, Luffy after two years can seemingly control where the air is stored within his limbs, where his fist becomes large and instead of the air being equally distributed throughout his arm, an equivalently large mass forms behind the fist. This allows a majority of Luffy's arm to remain as it is, permitting him to stretch it, thus allowing him to attack and withdraw much faster than pre-timeskip. He was seen being able to enlarge his right fist while inside a Yarukiman bubble (where there is limited air supply) rapidly upon engaging against the Kraken, demonstrating the use of infusing Busoshoku Haki into his colossal-sized arm to enforce its durability and defeat the sea monster. Luffy is confirmed to have overcome his weakness of shrinking when deactivating this Gear, after inflating both his arms and using "Gomu Gomu no Elephant Gatling" on Noah for a prolonged period of time and not turning into his chibi form.
Luffy has also shown to use this form defensively in battle, as seen when he fought Admiral Fujitora. In this battle Luffy enlarged his arm using Gear Third (though the enlargement was comparatively small compared to usual), and applied Busoshoku hardening to it in order to use it like a shield against the Admiral's powerful attacks. In this way, it was capable to defend against Issho's slashing attacks.
Most of the techniques in this mode are named the same as the regular attacks with "Gigant" added after the "Gomu Gomu no" prefix. "Gigant" is German for "Giant". In some techniques like Gigant Whip and Gigant Axe, Luffy uses the English name of the technique instead of the Japanese name, saying "Axe" and "Whip" instead of "Ono" and "Muchi". The known named techniques that have been used by Luffy so far in this form are as follows:
Hone Fusen (骨風船,Hone Fūsen?, literally meaning "Bone Balloon"): Luffy inflates his body by blowing air into his bones. This is done by biting into his right thumb, presumably cutting it open down to the bone, and then blowing air directly into the opened bone. This technique seems to give Luffy a sturdier defense than the regular balloon, since bullets now ricochet off of him instead of bouncing off of him. Due to this form having a disadvantage of moving about, he can transfer his air into his torso, and then launch himself with his "Gomu Gomu no Rocket" technique. This was first fully shown in Luffy's battle against Rob Lucci. In the Viz Manga and FUNimation dub, this is called Bone Balloon.
Gomu Gomu no Gigant Pistol (ゴムゴムの巨人の銃,Gomu Gomu no Giganto Pistoru?, literally meaning "Rubber Rubber Giant Pistol/Gun"): Luffy delivers a Gomu Gomu no Pistol with extreme force due to the larger size of his fist. This was first fully seen being used against Rob Lucci. Its sheer power was able to fling him from the Bridge of Hesitation on to a Buster Call Battleship. In the Viz Manga, FUNimation dub, and FUNimation subs, this is called Gum-Gum Giant Pistol. The kanji's literal reading is "Kyodai no Juu".
Gomu Gomu no Rocket (ゴムゴムのロケット,Gomu Gomu no Rokketo?, literally meaning "Rubber Rubber Rocket"): A Gear Third version of Luffy's normal Gomu Gomu no Rocket. While using this move, Luffy's chest bones are inflated, and bullets ricochet off his body. This is not explicitly named as a "Gigant" move. This was first seen being used to propel Luffy onto a Buster Call battleship in pursuit of Rob Lucci.
Gomu Gomu no Gigant Axe (ゴムゴムの巨人の斧,Gomu Gomu no Giganto Akkusu?, literally meaning "Rubber Rubber Giant Axe"): A Gomu Gomu no Ono with even more power due to his enlarged foot and leg. This was first seen being used against Rob Lucci. Its sheer power is able to damage and create a hole in the deck of a Buster Call Battleship. In the Viz Manga, FUNimation dub, this is called Gum-Gum Giant Axe. The kanji's literal reading is "Kyodai no Ono", and like the Jet version of Ono, it lacks the "sen" (war) kanji (戦).
Gomu Gomu no Gigant Whip (ゴムゴムの巨人の鞭,Gomu Gomu no Giganto Wippu?, literally meaning "Rubber Rubber Giant Whip"): Gomu Gomu no Muchi with more power due to his enlarged foot and leg. This was first seen being used against Rob Lucci. Its sheer power is able to damage the mast of a Buster Call Battleship. In the anime, Luffy used a haki-infused version against Doflamingo. In the Viz Manga and FUNimation dub, this is called Gum-Gum Giant Whip. The literal kanji reading is "Kyodai no Muchi".
Gomu Gomu no Gigant Bazooka (ゴムゴムの巨人のバズーカ,Gomu Gomu no Giganto Bazūka?, literally meaning "Rubber Rubber Giant Bazooka"): A Gear Third version of Luffy's Gomu Gomu no Bazooka attack. To create this attack, Luffy inflates both his arms by biting both his thumbs and blowing into them. With both his arms inflated to large proportions, Luffy then delivers the attack onto an opponent the same way he regularly does with a normal Gomu Gomu no Bazooka attack, except with much greater power. This was first seen being used against Oars to break his spine. In the Viz Manga and FUNimation dub, this is called Gum-Gum Giant Bazooka.
Gomu Gomu no Gigant Jet Shell (ゴムゴムの巨人のＪＥＴ砲弾,Gomu Gomu no Gigant Jetto Sheru?, literally meaning "Rubber Rubber Giant Jet Shell"): This was Luffy's strongest technique at the time. While he is still in Gear Second, Luffy goes into Gear Third with Hone Fusen. With the combination of Gear Second's high speed and Gear Third's air within his bones, Luffy launches his air filled body and rams into an opponent like a large cannonball. This move is quite reckless, as Luffy suffers both from the excess burn on his body from Gear Second, and the difficulty of moving his body from Gear Third. This was first seen being used against Gecko Moria while he is in his Shadow's Asgard form. In the Viz Manga and FUNimation dub, this is called Gum-Gum Giant Jet Shell. The literal kanji reading is "Kyodai no JET Houdan", with "houdan" being a Japanese term for a gun shell.
Gomu Gomu no Gigant Rifle (ゴムゴムの巨人の回転弾,Gomu Gomu no Giganto Raifuru?, literally meaning "Rubber Rubber Giant Rotating Bullet"): Luffy activates Gear Third to expand one of his arms and then use it to launch a giant version of the original Gomu Gomu No Rifle at a target with greater power, drilling hard into the unfortunate target. This attack is strong enough to destroy the heavily-armored Pacifista, and was first seen being used to defeat PX-4 on the Sabaody Archipelago. The fist produced in this attack is approximately the same size as the entire body of the giant Luffy uses it to defeat. In the Viz Manga and FUNimation subs, this is called Gum-Gum Giant Rifle. Unlike the variations of Jet Rifle, this version of the attack is written in kanji like the original base version; the literal kanji reading is "Kyodai no Kaitendan/Kaitendama".
Gomu Gomu no Gigant Stamp (ゴムゴムの巨人のスタンプ,Gomu Gomu no Giganto Sutampu?, literally meaning "Rubber Rubber Giant Stamp"): Luffy activates Gear Third and expands his foot to deliver a giant version of Gomu Gomu no Stamp with greater force. Incidentally, this was first used in conjunction with Mr. 3's Giant Candle Wall to push it back. This was first seen being used against Magellan in Impel Down. In the Viz Manga and FUNimation dub, this is called Gum-Gum Giant Stamp.
Gomu Gomu no Gigant Fusen (ゴムゴムの巨人の風船,Gomu Gomu no Giganto Fūsen?, literally meaning "Rubber Rubber Giant Balloon"): Luffy activates Gear Third and expands his torso to create a giant version of his traditional Gomu Gomu no Fusen, with a much sturdier defense. This move was strong enough to cushion the impact from a massive adversary and was first used to protect Ace and Mr. 3 from a punch of Sengoku in his giant form. In the Viz Manga and FUNimation dub, this is called Gum-Gum Giant Balloon. The literal kanji reading is "Kyodai no Fuusen".
Gomu Gomu no Gigant Gatling (ゴムゴムの巨人の銃乱打,Gomu Gomu no Giganto Gaturingu?, literally meaning "Rubber Rubber Giant Gun Battering"): Luffy activates Gear Third and expands both his fists to deliver a giant version of his powerful Gomu Gomu no Gatling technique. This technique is completely exclusive in the fighting game, Jump Ultimate Stars (using Luffy's strongest battle set a la his 8 Koma version, and acts as this version's Special Move A) and One Piece: Pirate Warriors (during the final fight against Akainu to deliver the final blow) but it does not appear in either the manga or in the anime. While appearing counter-intuitive with using increased mass and rapid-firing (especially since in Jump Ultimate Stars, Luffy does not move forward as much during the attack), the idea was incorporated post-timeskip. The kanji's literal reading is "Kyodai no Juuranda".
Like his normal and Gear Second forms, Luffy named his new techniques (after the timeskip) with a gun and artillery concepts theme related to an animal. Having learned to make the air "sit" in a specific part of the body, his Gear Third attacks are now both faster and stronger than they were before the timeskip. Furthermore, he no longer shrinks after using these attacks.
Gomu Gomu no Elephant Gun (ゴムゴムの象銃,Gomu Gomu no Erefanto Gan?, literally meaning "Rubber Rubber Elephant Gun"): The Haki enhanced version of Gomu Gomu no Gigant Pistol. After activating Gear Third, Luffy will also implement Busoshoku: Koka into his attack, turning his arm black. Just like his Gomu Gomu no Bullet, Luffy launches his attack at his enemy with great destructive power. However, the Gear Third's air only fills the end of his arm, giving him a giant fist with an elephantine round wrist. It is named after an elephant gun, a large caliber firearm used to hunt elephants that used black powder at first and then started using smokeless powder. This attack was first seen underwater against the Kraken. In the Viz Manga and FUNimation dub, this is called Gum-Gum Elephant Gun. The kanji's literal reading is "Zoujuu".
Gomu Gomu no Elephant Gatling (ゴムゴムの象銃乱打,Gomu Gomu no Erefanto Gatoringu?, literally meaning "Rubber Rubber Elephant Gun Battering"): After enlarging and imbuing both his arms with Busoshoku Haki in a similar manner as Gomu Gomu no Elephant Gun, Luffy performs his Gomu Gomu no Gatling, which, with the heavily enforced arms, causes massive damage. Since the bulk of the air sits in his forearms and fists and not in his upper arms, it allows him to punch and pull back rapidly. This technique was first seen to defeat Hody Jones, as well as breaking apart the falling Noah. In one of the specials, it was shown that he can also perform this technique with a single arm while retaining enough power to cause extensive damage to a large Marine base. In the Viz Manga and FUNimation subs, this is called Gum-Gum Elephant Gatling. The kanji's literal reading is "Zou Juuranda".
Gomu Gomu no Grizzly Magnum (ゴムゴムの灰熊銃,Gomu Gomu no Gurizurī Magunamu?, literally meaning "Rubber Rubber Grizzly/Ash Bear Gun"): A Haki enhanced version of Gomu Gomu no Gigant Bazooka. Luffy gets distance away from his opponent and activates Gear Third on both his arms simultaneously. He then runs towards the target with both his arms still static behind him before leaping in the air. While flying toward the enemy at a great speed he implements Busoshoku Haki into his arms, turning them black. He retracts his arms, and launches his attack with a great destructive power. The technique is named after the Grizzly Win Mag made by the gun manufacturer L.A.R. This attack was first seen being used to defeat Caesar Clown. In the Viz Manga and FUNimation dub, this is called Gum-Gum Grizzly Magnum. The kanji's literal reading is "kaiyuujuu"; it should be noted that the kanji used for "ash bear" is the Chinese term for a grizzly bear, and is not applicable to Japanese as the proper Japanese term would instead be "Hai'iroguma" (灰色熊, lit. Ash-Colored Bear).
Gomu Gomu no Thor Elephant Gun (ゴムゴムの雷将象銃,Gomu Gomu no Tōru Erefanto Gan?, literally meaning "Rubber Rubber Thunder General Elephant Gun"): An enhanced version of Gomu Gomu no Elephant Gun, first seen when Luffy used it to defeat Chinjao in the Corrida Colosseum, restoring the latter's head to its original drill-shaped form in the process. The attack differs from a regular Elephant Gun in that Luffy launches himself into the air with Gomu Gomu no Rocket first, before striking, discharging electricity when it lands. The attack is named after the Norse hammer-wielding god Thor. Thor in a technique name happened for the first time in the One Piece Film: Strong World, where it was a lightning strike enhanced Gigant Axe. The kanji's literal reading is "Raishou Zoujuu".
Gomu Gomu no Gigant Tsuppari (ゴムゴムの巨人のつっぱり,Gomu Gomu no Giganto Tsuppari?, literally meaning "Rubber Rubber Giant Thrust"): Luffy activates Gear Third and create a giant version of his Gomu Gomu no Tsuppari, which was also used in Luffy's sumo match against Urashima. Luffy stretches his arm very far back, and as it shoots forward he inflates it greatly before striking his opponent with a massive open palm. The palm is large enough to hit the entire body of the very large Urashima and send him flying out of the ring, destroying every building in his way.  This is called Gum-Gum Giant Sumo Slap in the VIZ Manga.
Gomu Gomu no Gigant Hae-tataki (ゴムゴムの巨人のハエたたき,Gomu Gomu no Giganto Hae-tataki?, literally meaning "Rubber Rubber Giant Fly Swatter"): After enlarging his hand, Luffy slaps his opponent with his palm. He first used this against a flock of Kung Fu Mynahs in Episode 542, the second One Piece x Toriko Crossover special. "Hae-tataki" (also written as 蝿叩き in kanji) is the term for a fly swatter, literally meaning "fly thrasher".
Gomu Gomu no Gigant Axe (ゴムゴムの巨人の斧,Gomu Gomu no Giganto Akkusu?, literally meaning "Rubber Rubber Giant Axe"): Luffy enlarges his leg and imbues it with Busoshoku Haki before slamming it down onto his opponent. It was first used against Charlotte Cracker's biscuit soldier.
Gomu Gomu no Grizzly Corkscrew Magnum (,Gomu Gomu no Grizzly Corkscrew Magnum?): A combination of Gomu Gomu no Grizzly Magnum and Gomu Gomu no Gigant Rifle. This attack was first seen being used in battle with Charlotte Katakuri.
Gomu Gomu no Gigant Stamp (ゴムゴムの巨人のスタンプ,Gomu Gomu no Giganto Sutampu?, literally meaning "Rubber Rubber Giant Stamp"): Luffy activates Gear Third and expands his foot to deliver a giant version of Gomu Gomu no Stamp with greater force and imbues it in Busoshoku Haki. This attack was first used against Kaido in his dragon form. This was first used in Episode 914.
Gomu Gomu no Gigant Jet Gatling (ゴムゴムの巨人のＪＥＴ銃乱打,Gomu Gomu no Giganto Jetto Gaturingu?, literally meaning "Rubber Rubber Giant Jet Gun Battering"): In One Piece: Gigant Battle! 2 New World, post-timeskip Luffy uses a variant of Gigant Gatling from Jump Ultimate Stars, mixing it with Gear Second, Busoshoku: Koka, and Elephant Gun. The kanji's literal reading is "Kyodai no JET Juuranda".
Gomu Gomu no Elephant Storm (ゴムゴムの象暴風雨,Gomu Gomu no Erefanto Sutōmu?, literally meaning "Rubber Rubber Elephant Storm (Rain)"): Used as his Secret Attack in One Piece: Super Grand Battle X. Luffy activates Gear Third and covers both of his arms with Busoshoku Haki, and combines both Gomu Gomu no Storm and Elephant Gatling. The kanji's literal reading is "Zouboufuu'u".
Gomu Gomu no Gigant Thor Axe (ゴムゴムの巨人の雷斧,Gomu Gomu no Giganto Tōru Akkusu?, literally meaning "Rubber Rubber Giant Thunder Axe"): A Gear Third move performed when Luffy enlarges his leg and thrusts it high into a low-lying thundercloud. When he brings it down, the charged limb unleashes a massive shock wave, powerful enough to cause the entire island underneath to crumble and be destroyed. Later, the lightning travels through giant cracks to give more damage. This was first seen used to defeat Shiki, and was only possible thanks to the storm that they were fighting in. The attack was named after Thor, the Norse god of thunderstorms, who wielded a hammer rather than an axe. It is also said that every time Thor hit with his hammer, a lightning bolt would be brought from the skies, not always hitting the opponent, but usually just for demonstrating its powers. The kanji is literally read as "Kyodai no Raifu". In the FUNimation dub, this is called Gum-Gum Giant Thor Axe.