@Shishou:
How can Gears be a changing form of his rubber body?
When Gear 2 showed no physical change. I mean, if the theory of Chibi Luffy being Gear 3 and not an after affect is true, then Gear 2 Luffy should be longer in all aspects. And not normal size.
So it would appear, Chibi Luffy is simply an after affect of Gear 3. And Gear 3 most obviously isn't an attack, but is an upgrade to his body that he can only keep up for a short amount of time. Think Kaio Ken from DBZ.
If going gear means denser more compressed rubber** then the different numbers to gear represent different levels of compression. And the higher the number the greater the level of compression. So in gear 2, he's still compressing his rubber, but not to the point of shrinking his rubber. I think Luffy's muscles in their relaxed state - his muscle tone if you will - were strong enough to counter-act any shrinking effects the rubber compression may have had in Gear 2. Gear 3 is a different story. In this super compressed state, his muscle tone couldn't keep his body from shrinking, and so you have chibi-Luffy. Also, just because his muscle tone couldn't maintain his original shape, it doesn't mean it's impossible for Luffy to stretch his rubber in Gear 3. Remember we're talking about Luffy's muscles in their relaxed state, not in their 'active' state (here I'd usually say contracting state - muscles contract when they're 'active' - but the physics of that, with Luffy's body the way it is, would be a little weird and nonsensical). So I've no doubt Luffy, with his superhuman strength, can still stretch his rubber (even though his rubber is now stiffer).
Also, I think Luffy holding his thumb in his mouth was a way of achieving greater compression. In gear 2, when we has compressing his rubber, all the steam and air was released in a haphazard unorderly way, making for reduced compression. But if he funnels all the air and steam through his thumb, sucking it out if you will, the steam and air release is more orderly, allowing for greater compression. Also in gear 2, he used only his muscles alone to release steam/air. With gear 3, along with his muscles, he also adds the sucking power of his mouth to help release steam/air, thereby achieving even greater rubber compression.
** for those not familiar (or still unclear) with the compressed rubber theory - compressed rubber increases tensile strength, which makes for stronger snap-back, meaning stronger and faster punches. It also makes rubber stiffer, thus hardening Luffy's body. However, because it's stiffer, more strength is required to stretch the rubber. I've no doubt Luffy has the strength to stretch gear compressed rubber. His stamina (how much energy he has to keep active in compressed form) however is not as clear.