Gomu Gomu no Leo Bazooka is a powerful technique that Monkey D. Luffy uses in the manga and anime series One Piece. This is one of the techniques that come under his category of Gear Fourth techniques, under the form of Boundman. Leo Bazooka is an empowered version of Luffy's regular old Gomu Gomu no Bazooka, strengthened by the properties of Gear Fourth.
First of all, Luffy inflates his arms in great proportion by blowing large amounts of air into the arms. In so doing, this drastic increase in size preps the arms for a very destructive impact. Then, he sends a double-handed fist technique with great might, connecting pure strength and elasticity from his rubber body.
This attack is endowed with a more consistent name, given Enel's fashion to naming with regard to Luffy's Gear Fourth arsenal. While basic "pistol" attacks in this form are named "Kong Gun" and "King Kong Gun", the Bazooka type attacks are consistently named after cats. In Leo's case, the name means lion, thus indicating an attack at its full strength and ferocity.
As such, there have been some rather unsurprising comparisons to the Leo Bazooka and other Gear Fourth techniques popping up in the fan discussions. It's pretty much just a big two-handed version of the Kong Gun, the argument goes, so it has to be stronger just because it uses both hands. That fits in kinda well with how power scaling in general does work in One Piece anyway, because two-handed attacks are usually more powerful than their one-handed imitations.
The Leo Bazooka has only been used in some of the most important fights—e.g., the battle between Luffy and Donquixote Doflamingo at Dressrosa. These include the proof of Luffy's growing power and his ability to fight stronger enemies by using advanced forms.
Note that the naming of Luffy's techniques has been translated differently across translations and dubs of One Piece. For example, while the name used in the original Japanese version for his attacks is "Gomu Gomu no" (), meaning "Rubber Rubber," the VIZ manga and Funimation adaptations use "Gum-Gum." The 4Kids dub went further and coined terms used for similar techniques, such as "Gum-Gum Double Barrel.".
Fans have also speculated what other future forms of this technique might appear to be. Some even believed in a "King Leo Bazooka" or some power-up form just like Kong Gun that became King Kong Gun. So far, in the story, no such technique has been shown.
In the context of Luffy's overall fighting style, the Leo Bazooka represents a considerable evolution of ability in him. It combines the raw power of Gear Fourth with the versatility brought about by his rubber powers to deliver devastating blows against opponents who would otherwise have been able to withstand his regular attacks.
If anything, its visual effect in manga and anime has been well-noticed; it is one of the techniques most sketched, fanned, and discussed by fans alike. Animated GIFs showing this attack and musical covers for Luffy's techniques—the Leo Bazooka are two things etched in the fanbase of One Piece.