Gamma Knife is an ultra-powerful and significantly specialized technique that Trafalgar D. Water Law wields in the popular manga and anime series One Piece. Among Law's attacks, this ability is reputedly one of his strongest and represents the high-level applications of his Ope Ope no Mi powers from the Devil Fruit. The technique is named after a real-world medicine treatment of the same name, in which focused gamma radiation is used to destroy tumors as part of precise treatments; thus, the idea is further set that Law comes from a background labeled as the "Surgeon of Death."
Gamma Knife is something from One Piece that can deal internal damage. In this move, Law constructs a short, razor-sharp blade of energy around his hand and strikes his opponent with it. Unlike most of his other cutting techniques, which involve the external parts of the body, Gamma Knife is specifically designed to inflict maximum damage on the internals of the target. It is because of this special property that this technique is especially useful against highly resilient or regeneration-adept enemies.
One of the most famous uses of the Gamma Knife in the series is during Law's epic battle against Donquixote Doflamingo, an opponent offered the better by the skills of his awakened Devil Fruit. This fight happened in episode 722 of the anime, titled "A Blade of Tenacity - The Gamma Knife Counterattack!", and it really showed how much destruction this technique could bring. Law caused major damage to the insides of Doflamingo with this technique and showed that it can penetrate any outer defenses.
The real power of the Gamma Knife lies in its effects on the opponent's body. Much like Haki can bypass a Devil Fruit user's abilities to various degrees and physically affect them, Gamma Knife is capable of inflicting damage that ordinary Devil Fruit powers cannot easily mitigate. This makes it especially potent against enemies who rely mainly on their Devil Fruit powers for defensive or healing purposes.
Though Gamma Knife is an overwhelmingly powerful technique, it is not invincible. For example, Doflamingo utilized his Awakened Devil Fruit powers to use his string abilities and "stitch" his internal organs back together from Law's attack, somewhat lessening the damage dealt. This case thus proved the strength of both Gamma Knife and Devil Fruit awakening in high-level battles within the One Piece world.
Gamma Knife, through the anime series, became an instant favorite with the audience. The eyes, mind, and imagination have been treated to a scene of Law—grasping an energy blade and using it to attack his opponent—often combined with the respective shot of what goes on inside. Already attaining such popularity, it has further spawned products such as collectible figures, including Figuarts ZERO One Piece Trafalgar Law-Gamma Knife, which was released in March 2018.
Gamma Knife is a very powerful Event card in the One Piece Trading Card Game, costing 2 DON!!. As in the series, it is extremely energy-consuming, returning DON!! cards in the field back into their DON!! deck. This makes it part of the card game as well, something which resonates even more on the viability of the technique in Law's arsenal and how much recognition this technique holds among fans.
Gamma Knife works similarly to some of Law's other electrical-based attacks, such as Counter Shock. All of these techniques involve the projection of massive electrical shockwaves throughout the target's body. Gamma Knife is more lethal than its cousins because it deals focused, inside damage.
Given that the One Piece series is still ongoing, a fan would look forward to seeing how Law may further develop or apply the Gamma Knife technique in future fights. Its unique properties and devastating effects ensure it holds a large role in Law's fighting style and remains a testament to his growth as one of the most compelling characters in the series.