|King Kong Gun|
Luffy using King Kong Gun.
|Effect||Luffy enters Gear Fourth to deliver a powerful, long-ranged punch.|
When performed, Luffy first uses Conqueror's Haki to create a black line of sight in front of him, paralyzing and dealing 10% damage to opponents hit. He then coats and inflates his arm to enter Gear Fourth before compressing his fist into his forearm. After reeling it back, he then delivers a powerful punch, which stretches out to cover a massive horizontal range. Opponents hit by the punch are dealt 31% damage with extreme knockback, which often is enough to one-hit KO. After the punch ends, Luffy returns to his normal form, ending the move.
Gum-Gum King Kong Gun is a powerful technique that Luffy uses in One Piece. It can be performed using the Gear Fourth, a technique in which Luffy coats his arm in Armament Haki and blows so much air into his body that he becomes massively inflated, particularly in his muscles. While using Gear Fourth, Luffy's strength, speed, and durability are greatly increased, but he can only use the form for so long before it automatically deactivates and leaves Luffy exhausted, in addition to him being unable to use Haki for ten minutes. Gum-Gum King Kong Gun is a technique Luffy uses in his first Fourth Gear form, Boundman, in which he compresses his fist into his enlarged forearm, blows more air into it to increase its size, and then unleashes a massively powerful punch. It is one of the most powerful techniques Luffy has ever utilized in any form.
The pose Luffy makes during the Final Smash is based on a pose he makes during his fight against Charlotte "Big Mom" Linlin in chapter 871 of the One Piece manga, in which Luffy attempts to land a punch with Gear Fourth that gets blocked. The kanji text that hovers over him if the attack connects reads "ドオン！！", a common sound effect that is found in the One Piece manga, including the aforementioned chapter, and in fighting games such as One Piece: Super Grand Battle! X and One Piece: Burning Blood.
- During development, it was heavily considered for Luffy's Final Smash to be based on his Ultimate Attack from the game J-Stars Victory VS, which uses Gear Third. However, this was changed following the introduction of Gear Fourth in One Piece.
|Luffy's special moves|
|Neutral special move||Gum-Gum Pistol|
|Side special move||Gum-Gum Rocket|
|Up special move||Gum-Gum Snap|
|Down special move||Gum-Gum Balloon|
|Final Smash||King Kong Gun|