Gear Second is the first of many Gears that Luffy developed in One Piece. After fighting Aokiji, Luffy figured that he had to get stronger quickly or not be able to protect any of his crewmates. Following that, Luffy and Zoro lost to CP-9. However, Luffy picked up the Soru technique from them.
Incorporating his Devil Fruit powers into it and using his rubber body to pump blood faster in his body allows Luffy to move much faster and strike much quicker. This form is so intense that all his sweat starts evaporating from his body as if it were steam.
Gear Third was developed by Luffy during the same time when he came up with the idea to use Gear Second. However, the application of this Gear is completely different. Unlike Gear Second, this Gear doesn't have much to do with speed and instead focuses on power. By biting into his thumb and inflating his bones like a balloon, Luffy can turn his body into that of a giant and deliver tremendously powerful blows. This technique was used against Rob Lucci for the first time, severely weakening him. After the time-skip, Luffy can use these techniques without any drawbacks.
Luffy can also use Gear Second and Gear Third to increase his mobility and get further stronger. This was first seen during his fight against Gecko Moria, where just one gear's power wasn't enough. As a result, Luffy used both the Gears to unleash a power known as Gum-Gum Giant Jet Shell.
Before the time-skip, it was said to have adverse effects on Luffy's lifespan. However, this doesn't seem to be the case as Luffy used them together many timesーincluding the time when he fought Yamato.
During the Thriller Bark arc, Luffy was fed around 100 shadows by those wandering the Thriller Bark to fight Oars and Moria. Once he consumed these shadows, Luffy grew tremendously in size, and his skin turned blue.
Although the power-up was temporary, it made him strong enough to stand up to Nightmare Luffy alone and beat him up with relative ease. Nightmare Luffy has never been used again after that arc.
Gear Fourth is Luffy's strongest known Gear at the moment, and it works on a principle similar to Gear Third, with the only difference being that Luffy inflates his muscles instead of his bones and uses Armament Haki.
One of the many forms of Gear Fourth is the Tankman, in which Luffy inflates his body so much that he looks like a stuffed tank. The full version of this power was used against Charlotte Cracker, and it proved to be strong enough to one-shot him. However, this form's biggest weakness is Luffy's lack of mobility due to his massive size.
Snakeman is one of the most powerful forms of Luffy, and it was first used against Charlotte Katakuri of the Big Mom Pirates. This form of Gear Fourth sees Luffy use his Armament Haki on his arms and his legs, for the most part, to keep his body as lean as possible to boost the speed further, as that is its main focus.
To fight the likes of Katakuri, Luffy needed to get faster, and according to him, Snakeman was fast enough to overcome Katakuri's Observation Haki. This form is incredibly fast, but in doing so, it loses out on the defensive edge that Bounce Man or Tank Man offers.
Bounceman is Luffy's go-to form in Gear Fourth at the moment, and it seems to be the best of them all in terms of raw strength. Bounceman makes the best use of the Muscle Balloon technique and uses maximum Armament Haki to make Luffy armored yet give him insane rubbery properties.
Bounceman also has some of the strongest techniques Luffy can use, such as the King Organ or even the King Kong Gun. While it isn't as fast as Snakeman, it makes up for it by offering a higher level of durability and also heavy-hitting blows. Without a doubt, Bounceman is Luffy's strongest form at the moment.