Monkey D. Luffy is the main protagonist of One Piece and then captain of the Straw Hats who dreams of becoming the ‘King of the Pirates’ and finding the One piece. Luffy accidentally ate the Gomu Gomu no devil fruit when he was 7 and it turned his body into rubber.
Over the course of years, Luffy has been able to train and develop his skills which has allowed him to create his Gears which are basically special techniques that are related to his devil fruit. Luffy has a total of 6 gears (Three being subforms of Gear 4) and are divided into four levels. These Gears have become stronger over time and give Luffy huge amounts of power.
(Please proceed carefully as there are spoilers in the article)
Gear 2 was first introduced during the Enies Lobby Arc. In this gear Luffy increases the speed of his blood and due to the rubber nature of his blood vessels and organs, his body can withstand the pressure. This allows his body to pump oxygen and nutrients throughout his body. It also increases Luffy's physical strength and gives him a lot of speed and defensive powers.
Luffy activates this mode by pumping blood throughout his legs to activate the mode throughout his body. Later on he can be seen to be able to activate Gear 2 on a selected body part. By combining his haki and Gear 2, he is able to punch his opponents with speed and force and this makes his fist turn into flames which Luffy named Red Hawk. Later on in the series, his Red Hawk became so strong that he could injure even Kaido during their battle in Onigashima.
Gear 3 was also introduced during the Enies Lobby Arc. This mode is activated when Luffy blows massive amounts of air into his thumb and can inflate the bones of a particular body part and make it huge. These body parts are capable of causing damage on a huge scale and can even destroy buildings. His power increases when he combines it with Busoshoku Haki and later on he even combines it with Haoshoku Haki.
At first Luffy used to turn into a child and his fighting abilities were limited temporarily after he used this mode. However, after the time-skip, where he spent two years training, Luffy has managed to negate this side-effect.
Luffy learnt Gear 4 during his 2 year training period and was seen using it during the Dressrosa Arc. Luffy activates this mode by blowing into his arm and inflates his muscles and then coats it with Busoshoku Haki. Gear 4 has three forms aka sub forms.
This form makes his skin composition like that of a rubber ball which causes him to bounce all the time. Luffy is able to retract his hands and legs which help him unleash devastating attacks and even change directions of his attacks. His defense also increases and the majority of the attacks bounce off him.
This form makes Luffy become much bigger and increase his defense abilities. This also allows him to use his body as a weapon and recoil from his enemies' attack and blast them away.
This is the last sub form of Gear 4 where Luffy turns a bit slimmer and focuses more on the offense. This form doesn't have defense abilities like the previous two forms. Despite that, he has a huge increase in attack speed and can change the direction of his attacks so that he can hurt his enemies from all directions.
However, as strong Luffy’s Gear 4 is, he is usually not able to use haki for ten minutes after this mode and using it a lot of times can take a huge toll on his body.
Gear 5 is the recent mode that Luffy activated but this has only occurred in the manga and has yet to be animated. This mode was a result of Luffy awakening his Devil Fruit for the first time since the Blank Era.
In this mode, Luffy has many new abilities like being able to manipulate the shape and size of his body instead of focusing more on hardening his body with Busoshoku Haki. He looks more cartoon-like while fighting. He is able to give his surroundings rubber-like properties also which helped him deflect Kaido's Bolo Breath. He is also able to stretch any object he touches. His power has the characteristics of both Paramecia and Zoan type Devil Fruit.
A. Gear 5 is Luffy's strongest gear.
A. Gear 2 is Luffy's weakest gear.