Gear Fourth - Tankman is a unique variation of Monkey D. Luffy's Gear Fourth transformation in the One Piece series. However, it goes more in line with other forms of Gear Fourth, in which include the power of Luffy's Gomu Gomu no Mi Devil Fruit enhanced with Haki; however, this emphasis that Tankman places is on being defensive and having high stamina, unlike the other forms of Gear Fourth focusing on another aspect of combat.
This Tankman made his first appearance in Episode 806 of the anime One Piece, titled "The Power of a Full Stomach - New Gear 4 Tankman!". An episode that truly unleashed the special abilities of this form and how it surprisingly showed up during the moment of crisis for Luffy's fight. The situation itself, where finally Tankman made an appearance, is also very interesting: at a time when Luffy had eaten too much food, which really made him look very bloated.
The visual design for Tankman differs from all of the other Gear Fourth variations. While forms like Boundman and Snakeman distort Luffy's body to grant him enhanced speed or striking power, Tankman drastically inflates his girth. That inflated appearance actually serves a functional purpose, enhancing Luffy's defensive capabilities while offering a unique offensive strategy.
One of its key moves is to absorb and then direct attacks. The very first episode it was introduced, Luffy used it against an actually very strong strike coming from the other side and proved that a form like Tankman would work well in turning an enemy's strength against them. With the additional feature of being high in stamina, all that is really a true powerhouse form to take on in long fights is Tankman.
However, the status of Tankman as a standard Gear Fourth form has been highly debated by fans. Some believe it's a legit, reusable transformation. Others think it is a one-time deal, at least in the context it first appeared. This vagueness has led to endless speculation regarding exactly what Gear Fourth is and how situational it could be.
In the course of One Piece, Tankman represents an interesting development in Luffy's arsenal. While forms like Boundman were shown earlier and used more frequently, Tankman shows that Luffy can adapt his powers into very unforeseen ways, mostly in reaction to immediate tactical needs.
Despite this rather downright shocking debut, he has not appeared as much in the later arcs or adaptations of One Piece. For instance, in the video game "One Piece: Pirate Warriors 4," fans pointed out that Tankman is nowhere in any form as a playable form; he could have been added as just one special move instead of always being transformed. This has furthered the argument as to where exactly Tankman holds in terms of Luffy's techniques, with the fact that he clearly lacks said techniques in their proper existence.
Worth noting, with the introduction of Gear Fifth, which shows an enormous power increase for Luffy, speculations were triggered as to whether Gear Fourth forms, such as Tankman, have become obsolete. On the other hand, the unique defensive properties of Tankman say it could still have situational utility even in the face of Luffy's newer and more powerful transformations.