The Gasu Gasu no Mi is a Logia-typeDevil Fruit that allows the user to create, manipulate, and transform into gas at will. "Gasu" (ガス,"Gasu"?) means "gas". In the Viz Manga and FUNimation subs, it is called the Gas-Gas Fruit. It was eaten by Caesar Clown.
This Devil Fruit's main strength is the ability to change into a gas-like form, allowing the user to float and go inside places that normal beings cannot, as he was seen hiding inside some flasks when he was informed that the Marines were nearby. However, he can also become dense enough to hold objects, like his cocktail glass. As explained by Brownbeard, he claimed to be the one who purified the whole island from poisonous and nerve gases. This however is revealed to be just a cover story, with the poisonous gases being used to give birth to Smiley.
The gas that Caesar produces is very poisonous. The gas appears to be explosive when set alight, which allows Caesar to create controlled explosions without harming himself. Additionally, the user has the power to manipulate other preexisting gases in any given location, and can suffocate their opponents by removing the oxygen in the area. This gives the user two options to instantly defeat their opponent, allowing Ceasar to knock out Luffy after realizing that Luffy was immune to poison (however the area effect is finite, so if someone escapes the oxygen-free zone they will be able to breathe again). They can even merge with such gases to enhance his own power, depending on the gas being absorbed, all the while being immune to the toxic effects himself.
The gas form appears to be weak against fire, although this is mitigated by the user's ability to minimize oxygen levels in the surrounding area, preventing combustion. On the other hand, it seems to take some concentration or is only possible while solid to manipulate the oxygen, as Usopp and Nami could breathe again when Brownbeard struck Caesar with a metal pipe, and there seems to be a range of which Caesar can control oxygen, leaving him more vulnerable to longer-ranged attacks. Also, many of Caesar's techniques require external ignition and tools to work. Other than that, the user is also affected by the standard Devil Fruit weaknesses.
Caesar Clown has mainly used his fruit as a means of stealth, slipping into flasks, and floating in the air. He has enough control over his fruit to move freely and seize solids such as a cocktail glass.
In battle, Caesar releases poisonous gases that seems to cause instant death to an average victim, or ignitable gases of tremendous explosive power. He can also absorb and remove the oxygen within his vicinity, removing his weakness against fire, as well as force his opponents into unconsciousness or even death via asphyxiation. Out of desperation, he absorbed the H2S gas from Shinokuni into his own body to increase his own size and strength.
Gas Robe (ガスローブ,Gasu Rōbu?): Caesar generates a cloud of poisonous gas and sends it towards his enemy. He claims to have conducted many experiments with the gas, to the point where he is sure that breathing it in will cause certain death. Despite this, the attack was ineffective against Luffy due to the poison immunity he acquired from Magellan.
Gastanet (ガスタネット,Gasutanetto?): Caesar fills a pair of castanets with unstable gas and slams them together to create a massive gas explosion. Despite the scale of the attack, it does not cause any harm to Caesar himself. When fighting Luffy, he was able to increase the strength of the attack by surrounding him with smaller globs of slime and detonating them, although Luffy was able to escape unharmed. The name of the technique is a pun on the castanets (カスタネット) used to perform it.
Karakuni (無空世界（カラクニ）,Karakuni?, kanji read as "Airless World", phonetically read as "Empty Land"): Caesar removes the oxygen from the area around him, asphyxiating everyone within range. The attack was first used to defeat Luffy in an instant, as well as Robin, Franky, Smoker and Tashigi, but it was not named, and it was not explained until he used it against Usopp and Nami while they were attempting to protect the rampaging children. The attack's name was revealed during his second fight with Luffy, in his laboratory. The technique is one of the most dangerous in Caesar's arsenal, as it was able to instantly defeat several powerful fighters in mere moments without touching them, and none of them were able to defend against it in any way. One of the G-5 Marines mistook this technique for Haoshoku Haki. Luffy avoided the effects of the technique in their second fight by keeping his distance from Caesar, and relying on long-ranged attacks. In the Viz manga, this is called Land of Nothing.
Nenshokei: Miok Gas (燃焼系 ミオークGAS,Nenshōkei: Miōku Gasu?, literally meaning "Combustion System: Miok Gas"): Caesar concentrates flammable gas in the area, then attacks by throwing lit matches, causing a big ball of fire to form and incinerate whoever is inside it. This attack was first used against Luffy during their second battle (inside the laboratory). In the FUNimation subs this is called Flammable Type, Miok Gas.
Blue Sword (青炎剣（ブルーソード）,Burū Sōdo?, literally meaning "Blue Flame Sword"): Caesar concentrates oxygen (the oxygen removed by Karakuni can be used for that) on a special weapon. This weapon is a bladeless sword that doubles a lighter. By activating the lighter, Caesar creates a blade of oxidized flame that can be used to slash and burn his opponents at the same time. The blue blade is so hot that a mere slash set everything in its arch on fire despite not making contact with the weapon. The attack was first used against Luffy in the laboratory.
Gastille (ガスティーユ,Gasutīyu?): Caesar shoots a beam of lit gas from his mouth that explodes on impact. According to one of his subordinates, the explosion can reach temperatures hot enough to melt steel. Its appearance and effects are similar to Kizaru's and the Pacifista unit's laser beam but wider and a different form of explosion. It was first used against Luffy, but he dodged it, resulting in part of the hallway melting instead. In the anime, the beam is blue, similar to the "blade" of his Blue Sword.
Shinokuni (シノクニ,Shinokuni?, literally meaning "Land of Death"): After absorbing the petrifying gas of the chemical weapon, Shinokuni, Caesar gets a new form. In this form his size increases dramatically, he gets more horns than usual, the color of his outfit is changed to the color of the gas and the kanji for "Death" (死) and "Land" (国) are written on his chest. The real power of this technique seems to be the ability to acquire the properties of the gas; when Luffy attacked Caesar in this form, his arms became coated with the poison, despite the attack itself defeating Caesar. This was first seen in his third fight against Luffy.
As stated by Caesar himself, he altered his own Devil Fruit with his chemical expertise, adding many of it's offensive abilities, thus, the original abilities of the Gasu Gasu no Mi remain uncertain.
Since this fruit's power revolves around gas and poisonous substances, it is very similar to the Moku Moku no Mi and Doku Doku no Mi. However, Caesar is shown to be personally offended when someone compares his Devil Fruit to the Paramecia-class Doku Doku no Mi.