Third Gear is another powerful form that is first hinted at during the climax of the fight between Luffy and Blueno, and first used, but not witnessed, to knock down a steel door in Enies Lobby. It was first fully seen in Luffy's fight against Rob Lucci.
Luffy corks his mouth with his thumb, bites into his thumb joint (making a small opening) and blows very hard into it, which inflates his arm. He is then able to transfer the air through his entire body, requiring the torso to act as an interface for his various limbs. Luffy blows so much air into his body that when they are all focused on a single limb, they become the size equivalent to (or greater than) that of a giant's, if anything resembling Oars' size. 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 trouble moving.
Third Gear is generally used when raw physical strength is needed, the bones are the key difference between this form and his old "Gum-Gum Balloon" move, where his balloon form was soft and squishy like a regular balloon, whereas the inflated bones are very hard, similar to an inflated car tire. Unlike the normal Gum-Gum Balloon technique, bullets ricochet off of Third Gear instead of bouncing off. The bones being much harder also mean a lot of compression of air takes place when using Third Gear, making Third Gear not just a "for-show" move as Lucci initially thought, but like a giant mace of astounding mass when used in attacks. Because of Luffy's raw strength, he is able to move his gigantic arm with ease.
In between his attacks, Luffy brings the air back to his chest and then shifts it into another body part. The downside is, the lag time in which it takes Luffy to transfer and re-channel the amount of air trapped inside his bones can be very disadvantageous.
Before the timeskip, while in Third Gear, it proved quite difficult for Luffy to move while part of his anatomy has been converted to gigantic proportions, making the Gear inadvisable to use against opponents capable of moving at high speeds (like Lucci) or in an enclosed place as he is leaving himself vulnerable to attack, which is why since his battle against Rob Lucci, Luffy tends to only activate Third Gear against massive and often slow-moving targets whom he likes to assault with his Third Gear moves coming from high above in mid-air, where the said attacks could not be obstructed. To deactivate Gear Third, Luffy leaves the form by exhaling heavily, which can cause him to be propelled backwards.
The deactivation of Third Gear brings about shrinking, which is a negative side-effect for Luffy due to the massive expansion undergone prior to release. The amount of time Luffy spends in his shrunken form is roughly equivalent to the amount of time spent in Third Gear mode. This shrunken form has shown little to no ability to stretch at all, and it has been shown that Luffy loses an incredible amount of strength when he kept hitting Lucci's face to no effect. The best strategy Luffy could use, while waiting for himself to return to normal, is to run and hide from an opponent for the duration of the shrinkage. Another strategy is only using Gear Third for a few seconds for only one move so his shrinkage is minimized. But during the Impel Down Arc, Luffy demonstrated using his Third Gear with great speed. Being fast enough to defeat Jailer Beasts, who possessed Soru-level speed. During the Whitebeard War, this drawback was not shown during every use, perhaps being glossed over by the many different battles taking place all at once.
After the timeskip, as with Second Gear, Luffy seems to have obtained better skill at utilizing Third Gear. Previously, Third Gear 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, Luffy after two years can seemingly control where the air is stored within his limbs, where his fist becomes large and instead of the air being equally distributed throughout his arm, an equivalently large mass forms behind the fist. This allows a majority of Luffy's arm to remain as it is, permitting him to stretch it, thus allowing him to attack and withdraw much faster than pre-timeskip. He was seen being able to enlarge his right fist while inside a Yarukiman bubble (where there is limited air supply) rapidly upon engaging against the Kraken, demonstrating the use of infusing Armament Haki into his colossal-sized arm to enforce its durability and defeat the sea monster. Luffy is confirmed to have overcome his weakness of shrinking when deactivating this Gear, after inflating both his arms and using "Gum-Gum Elephant Gatling" on Noah for a prolonged period of time and not turning into his chibi form.
Most of the techniques in this mode are named the same as the regular attacks with "Giant" added after the "Gum-Gum" prefix.