The Giro Giro no Mi is a Paramecia-type Devil Fruit that allows the user to see through everything and read the minds of others, allowing the user to become an Insight Human (眼力人間,Ganriki Ningen?). It was eaten by Viola.
The main strength of the fruit, as demonstrated by Viola, is that it allows the user to see through everything, both literally and figuratively. By forming circles out of their index finger and thumb and putting their hands next to their eyes, like glasses, the user can see through a person's clothes and skin, much like X-ray specs, but also read their minds and memories, allowing them to see through any lies. It also allows the user to transfer their own memories to another person's mind, by letting another person look through the circled fingers.
The fruit also has offensive capabilities, as it allows the user's tears to be manipulated and turned into dangerous projectiles.
While there are no obvious weaknesses, it seems like the thoughts of a person can shock and disturb a user without preparation (Such as Sanji's powerful obsession of beautiful women). A fortitudinous mind is also required, as the uses of the fruit such as telepathy can cause emotional or mental damage with prolonged use. Aside from that, the user is affected by the standard Devil Fruit weaknesses.
Viola basically uses her ability to see through her targets, whether it be clothes or mind, or allow her targets to see into her memories. The value of this ability is such that Donquixote Doflamingo accepted Viola's offer to spare her father's life in exchange for her services.
Peeping Mind (心覗き,Pīpingu Maindo?, literally meaning "Heart Peeping"): After activating her hand glasses, Viola touches her head to the head of another person, allowing her to look into the person's mind and memories. Viola is also able to use this technique in reverse, allowing her to transfer her memories to the person.
Hierro Lágrima: Mekujira (熱鉄の涙目鯨,Iero Ragurima: Mekujira?, literally meaning "Hot Iron Tear: Eye Whale"): Viola secretes one teardrop from each of her eyes. The tears then grow into large whales that move to attack whomever they are aimed at by the user. Hierro Lágrima is Spanish for "Iron Tear", while Mekujira is Japanese for the corner of one's eye, though the specific kanji reads as "Eye Whale", hence the technique's pun. In the Viz manga this is called Hierro Lagrima Tear Whales.
Senrigan (千里眼,Senrigan?, literally meaning "Clairvoyance"): This aspect of her fruit allows Viola to send out her sight in all directions in a 4000 km radius like a bird. The ability was first mentioned by Gladius, who noted that it was strange that Viola failed to locate the Thousand Sunny as it approached Dressrosa. The extraordinary range of the technique allowed Viola to see everything that was going on in the country. The kanji for the ability's name together mean "clairvoyance", but separately they can be read as "thousand ri sight", where a ri is a unit of measurement in Japan equal to 3927.27 meters. Accordingly, a thousand ri is almost 4000 km.