DEVIL FRUIT

     Luffy has gained extraordinary stretching powers from eating the Gomu Gomu no Mi (lit. Rubber Rubber Fruit, Gum-Gum Fruit in the English dub), which is one of the rare and legendary Devil Fruits. After consuming the Gomu Gomu no Mi, Luffy's body became like rubber, giving him the ability to stretch long distances. Having trained hard for ten years, Luffy is able to supplement his stretching powers with immense strength, agility, and creativity in combat. His fighting style is largely improvised, and heavily reliant on his Devil Fruit. It should be noted, Luffy can't automatically stretch his limbs (like superheroes with stretching powers, i.e: Mr. Fantastic or Plastic Man) since his body acts more like a rubber band, thus has to throw some force into it in order for his limbs to stretch out. However, he can hold his limbs outstretched for a long time, as seen when he reaches his arm out about half a mile to retrieve Usopp from Water 7.

     Luffy uses his stretching power in ingenious ways. He is able to use the malleable stretchiness of his limbs to gain tremendous velocity and hit enemies with devastating impact. His rubber body is also extremely durable and virtually immune to blunt objects such as bullets (in One Piece, bullets are still round pellets), punches and electricity, bouncing off his body harmlessly or cancelling out attacks. It's even become a running gag that people find out the hard way Luffy is made of rubber when they shoot at him, only to have the bullets bounce right back. Another running joke is that whenever Luffy's body is in peril that would kill a normal human, he says that he's made of rubber, so it's okay, or acts like something was a small problem despite the person trying to kill him, such as being shot in the heart. However, he is still normally susceptible to sharp objects such as swords and knives as well as nonphysical attacks like heat, poison, cold, and dehydration. Not only can he use his ability to increase the range and strength of his attacks, but he can also use it to restrain and even control foes like puppets.Gear Second (ギア･セカンド, Gia Sekando) is where Luffy uses his legs to pump blood around his body at a much faster rate. Activating Gear Second also uses a lot of stored energy from his body. Leaving his body exhausted, his heart rate also increases to levels that are normally impossible. His veins are able to withstand this, thanks to the Gomu Gomu no Mi. In this form, Luffy gains a considerable boost in strength. During the Enies Lobby Arc, Rob Lucci mentions that this is just like steroids because his muscles are receiving enough blood to do things otherwise impossible. Rob Lucci also noted that Gear Second does damage to Luffy's own body and in turn drastically reduces his stamina and his lifespan begins to diminish.

     Luffy first discovers this Gear unofficially in the 7th One Piece Movie, where he has a large mechanical arm falling on him, he catches it, and the impact exerts extreme force on Luffy's legs, causing him to unintentionally pump blood at an accelerated rate to perform Gear Second and use the Gomu Gomu no Jet Bazooka (though he simply refers to is as a normal Gomu Gomu no Bazooka) to defeat a giant Mecha.

After the time skip, it appears that Luffy has gained enough control over his Gear Second form, in that he is able to control his targeted area rather than the whole body. This in conjunction with his Busōshoku Haki was able to take out a Pacifista with a single punch, something that took the entire Straw Hat crew to do before the time skip. It seems that Luffy has become more adept with this gear as he has been using Gear Second attacks more frequently than before the time-skip. Gear Third (ギア･サード, Gia Sādo) is where Luffy pumps air through his bones to make them literally gigantic. Obviously, only Luffy can do such a thing by using the Gomu Gomu no Mi. Used with his already-strong physiology, this apparently gives his arm more mass and a larger area to attack with far greater strength, but he sacrifices mobility for this strength, as his limbs are now so large that Luffy has extreme trouble moving unless he is in his Fuusen form. Another after effect of this is that Luffy is reduced to child-sized proportions, the duration of which is equal to the time he spent in Gear Third. Also, he is able to use both gears at the same time which he uses to defeat Gecko Moria. This technique is first hinted at during Luffy's fight with Blueno, and first used, but not shown when Luffy is chasing Rob Lucci and Spandam, but is not seen until a later battle with Rob Lucci.

     After the time skip, Luffy's Gear Third is vastly improved upon. Previously, Gear Third would increase the size of the majority of Luffy's arm, thus reducing the speed of the attack in exchange for a powerful impact. However post-timeskip Luffy can seemingly control where the air is allowed to "sit" within his limbs, where his fist becomes large but allows a large mass to form behind the fist as the equivalent of the mass he would have achieved having his arm fully inflated. This allows a majority of Luffy's arm to remain as it is, allowing him to stretch it, thus allowing him to attack much faster than before the two-year absence.

     Luffy is also shown using Gear Third at the same time as Busōshoku Haki causing his arm to turn black and harden (possibly undergoing vulcanization), with his arm in the ocean (which would normally drain a Devil Fruit user of their power). While he is affected by the ocean, he still lands a punch which knocks the Kraken out. After the attack (Gomu Gomu no Elephant Gun), Luffy appeared to remain the same size without shrinking temporarily, indicating than he may have overcome this side effect at some point during his training. Or like in the manga version of Marineford, Oda skipped it.