Gear 2
We know Gear 2 works by Luffy pumping his rubber body and increasing the flow of blood to allow him to fight on another level. It increases his speed, strength and I assume durability. It’s akin to doping. In Gear 5 we see Luffy do something pretty much the same with the sound of his heartbeat. Luffy is capable of changing the rhythm of his heart to pump himself with more energy and restart Gear 5.
The Stances are identical and the smoke is there.
Interestingly enough, not only is this very similar to what he did in his fight with Lucci and the stance he takes while doing so is very reminiscent of his original Gear 2 stance, but Lucci and Kaido’s comments about Luffy killing himself by doing so are early similar.
If it isn’t obvious I’m trying to say is that Luffy’s Gear 2 is his attempt to replicate this ability but being limited by his elasticity.
Gear 3
I think this is pretty obvious. In Gear 3 Luffy can blow air in his body to make it bigger. Originally, this ability had a drawback of making him chibi but after training during time-skip this drawback was no more. Post time-skip Luffy can freely manipulate his body by blowing air into it.
In Gear 5 Luffy no longer requires air to enlarge his body parts. Not only can he freely manipulate the size of his limbs (no blowing air needed) but he can also enlarge himself completely and turning himself into a giant. Best thing is that so far, there are no drawbacks to this ability (at least no chibi Luffy for some time).
Gear 4
Introduced to us in Chapter 783 (784 to be exact) Gear 4, Bound (Bounce) Man immediately stuck out as a sore thumb. It was so different in appearance from everything we have seen before. I believe this is the earliest that we can say for sure that Oda had the plan to make Luffy’s fruit something else. Interestingly enough, the reveal of Luffy’s Gear 4 is when I first started seeing theories about Luffy’s fruit not being Gomu Gomu no Mi. The uniqueness of Gear 4 didn’t stop here as not too long after we were introduced to two other forms of it with Tankman and Snakeman.
Not only did Luffy’s Gear 4 transformation look like a Zoan transformation but he also had different forms of it akin to Chopper and his Points. Also funnily enough, each of his Gear 4 forms had “-Man” in their name which is funny in hindsight because we now know his fruit is actually “Human Human Fruit”.
The biggest addition Bound Man brought to Luffy’s arsenal are his inflated muscles. By covering his body in Armament Haki and blowing air into his muscles Luffy was able to inflate the size of them so he can deal bigger damage to his enemies (according to Luffy he did so to fight giant animals).
If Hyogoro thought Luffy looked like a Wisdom King in his Gear 4th form he should see him now! He is the spitting image of a Wisdom King.
In Gear 5 Luffy is capable of inflating his muscles on will, without having to blow air into them, and now not only is he covering his body in armament but according to Kaido he is also adding Conqueror’s Haki on top of it.
It seems like Luffy fully utilized everything he has learned up until now to create his Gear 5. It’s not just his Fruit’s Awakening that makes the Gear 5 but also every Gear that came before it and everything Haki related he learned after his fight with Katakuri.
Luffy’s Gear 5 is him using his previous gears but dialed up to 11.
