Gear 5 ("Gear Fifth") is a transformation technique created as a result of Luffy awakening the power of the Gomu Gomu no Mi. It was first seen during Luffy's third battle with Kaidou on the Skull Dome rooftop.
The awakening of the Gomu Gomu no Mi, formerly known as the Hito Hito no Mi, Model: Nika, grants the user's rubber-like body increased physical strength and freedom, limited only by the user's imagination. It is for this reason that the user is said to become the "Warrior of Liberation" (解放の戦士,Kaihō no Senshi?), able to fight however they please and bring joy to those around them, making it "the most ridiculous power in the world". One of the most notable features of this fruit's awakening is its bestowal of cartoon-like effects, not only to the user but also to multiple people who are close enough to see and/or be around the user, including their opponent, an example being eye-popping effects, and cartoonish welts after a head injury; also, while in this state, the user displays a grand state of elation, laughter and confidence.
In this state, Luffy's heartbeat takes on a musical rhythm which has been described by Zunesha as the "Drums of Liberation" (解放のドラム,Kaihō no Doramu?). His hair and clothes turn white, his eyes glow with white ring-like pupils, and white clouds float around his neck, similar to the clouds of steam that accompany him in Gear Fourth. The amount of Haki released from this form was enough that it could be felt all the way to the Live Floor in the Skull Dome, knocking multiple Beasts Pirates out in the process.
Aligning with the Mythical Zoan nature of this new form, Luffy's already rubbery body gains strength, freedom and durability that surpasses what he would receive from his previous forms, to the point that it is now even cartoonishly malleable, with Kaidou likening it to "something out of a picture book". An example of this is when Luffy took a direct hit from one of Kaidou's Bolo Breath attacks, only to come out of it being barely hurt, his body being charred (somewhat comedically) and then him running back to fight Kaidou. Another example is when he reactivated Gear 5 and took a hit to the head from Kaidou's Hassaikai, only for his head to warp and wrap around the weapon and its spikes, or when Kaidou tried to smash him into the ground, his body was simply squished instead and later returned to its original shape.
Luffy is able to utilize both the limb inflation of Gear 3 and the muscle inflation of Gear 4 without the need to pump air into his body first, which he first used against Kaidou. He was able to increase his arm to an enormous size in order to grab Kaidou's transformed body and yank him up out of the Skull Dome. With his enlarged limbs and muscles, Luffy could easily manhandle Kaidou despite his enormous size, swinging him from side to side. Luffy can also perform the inflation over his entire body at once, transforming himself into a giant of massive size.
Luffy's rubbery nature can be extended into the environment around him, as seen when Luffy stretched a section of roof and used it to bounce Kaidou's Bolo Breath back at him. Interestingly, the altered terrain, while elastic, was neither burnt nor melted by the Bolo Breath, even though the attack was shown to be scorching hot and had enough explosive power to destroy a castle and a mountain top. Luffy can affect his surroundings to the point where he can make surfaces like the ground beneath him rubbery to cushion himself from powerful blows and even bounce off and launch himself off of whatever surface his power effects. This power also seems to be able to effect organic material as well, as shown when Kaidou literally swallowed Luffy whole, and then proceeded to have his body expand and inflate along with Luffy's. This, coupled with Luffy's own transformed state, has been pointed out by Kaidou to be very unusual, due to featuring elements of both Paramecia and Zoan awakenings.
Despite the strength of this form, due to inexperience, it consumes plenty of energy to maintain and use, at which point Luffy will visibly age once de-transformed, and also experience an extensive amount of exhaustion and fatigue. That said, Luffy can transform back by seemingly forcing his heart to beat in the rhythm of the Drums of Liberation. In addition, Luffy still has his standard weakness to cutting attacks.
Gomu Gomu no Fusen (ゴムゴムの風船,Gomu Gomu no Fūsen?, literally meaning "Rubber Rubber Balloon"): A variant of Luffy's standard technique, unlike the original technique, Luffy's body becomes extremely buoyant, like a helium balloon. This was first used against Kaidou in his dragon form, where Luffy entered the Emperor's body and expanded himself as well as the dragon. In the VIZ Manga, this is called Gum-Gum Balloon.
Gomu Gomu no Gigant (ゴムゴムの巨人,Gomu Gomu no Giganto?, literally meaning "Rubber Rubber Giant"): A technique where Luffy expands himself to the size of a literal giant, as if activating Gear 3 on his entire body. The name of the technique follows the Gear 3 naming theme, but without anything after "Gigant". In the VIZ Manga, this is called Gum-Gum Giant.
Gomu Gomu no Bajrang Gun (ゴムゴムの猿神銃,Gomu Gomu no Bajurangu Gan?, literally meaning "Rubber Rubber Monkey God Gun"): The Gear 5 version of Gomu Gomu no King Kong Gun, where he inflates his fist almost the size of Onigashima, infusing it with Busoshoku and Haoshoku Haki; then unleashes it at his opponent with the intention of punching them without making direct contact, through emitting Haki outward, for a devastating blow. This was first used against Kaidou, who countered the attack with Shoryu: Kaen Hakke, resulting in a massive clash. It was soon able to overpower Kaidou's technique, sending him crashing into the ground. "Bajrang" is an alternate name of the Hindu Monkey God, Hanuman, a potential inspiration for Sun Wukong. In the VIZ Manga, this is called Gum-Gum Bajrang Gun.
Gomu Gomu no Dasshutsu Rocket (ゴムゴムの脱出ロケット,Gomu Gomu no Dasshutsu Roketto?, literally meaning "Rubber Rubber Escape Rocket"): A technique used by Luffy to escape from inside Kaidou's body after using "Gomu Gomu no Fusen". Luffy stretches his arms through his opponent's eyes and grabs onto their nose to launch himself out. In the VIZ Manga, this is called Gum-Gum Escape Rocket.
Gomu Gomu no Nawatobi (ゴムゴムの縄跳び,Gomu Gomu no Nawatobi?, literally meaning "Rubber Rubber Skipping Rope"): The giant-sized Luffy grabs Kaidou in his dragon form and uses Kaidou's serpentine body as a skipping rope. In the VIZ Manga, this is called Gum-Gum Jump Rope.
Gomu Gomu no Kaminari (ゴムゴムの雷,Gomu Gomu no Kaminari?, literally meaning "Rubber Rubber Lightning"): Luffy gives a lightning bolt the properties of rubber in order to grab it, then hurls it at his opponent. This was first used against Kaidou, who managed to dodge the attack. In the VIZ Manga, this is called Gum-Gum Lightning.
Gear 5 was first hinted at in SBS Volume 98, when a fan asked if it was coming, and Oda responded by surreptitiously noting that the world was in the process of transitioning to 5G broadband networks.
In this form, Luffy's eyebrows become swirled and billowing, similar to Kaidou's in his dragon and hybrid forms.
Prior to the official introduction of the "Gomu Gomu no Gigant" ability, Luffy was previously shown as a giant in an anime filler episode with the aid of an ally, Lily Enstomach.