@Khon:
I have many reasons to believe Luffy's usage of Gears Second and Third is coming to an end.
http://www.onemanga.com/One_Piece/483/11/
Literally, he says:
"Don't you think Luffy's new fighting style is too harmful for his body? If there are even stronger enemies ahead, that means he'll have to keep doing this. I am worried…"
Again, the author Oda insists upon this point. Gear Second isnt something you can use as Dragon Ball's Super Sayajin transformation. It's not a simple "power-up". It's a hack, a doping, and a gamble. You risk your own health and life, gambling that your enemy will fall before you do. However, that cant go on forever. After using it often (and Luffy is using more and more often, see the fight with PX-1 and real Kuma) the effects are going to damage the body to a point it will, inevitably, be destroyed, rubber or not.
…
My point is. Both Gears have permited Luffy to get this far. Without them, the Mugiwaras would have perished long ago, in the hands of Rob Lucci or the Shichibukai Moria. However, it has proved uneffective against Kuma, and we cant expect Luffy to remain using Gear Second and Third every time he has to fight in the New World.
It's time to change. Luffy will manage to find a new way of fighting, and my bets are that he will learn the ways of "Ambition", also called Haki in the manga. It's probably what the old "players" use, as the Yonkou, Admirals and legendary pirates as Roger and Raylegh. This ability do probably allows them to place hits on Logia Devil Fruit users, even not weakening (as seastone) or nullifying (as Blackbeard) their powers.
However, this isnt a topic about the ways "Ambition" can be used, but as to discuss if this idea is valid or not, given the proves shown. I think Oda has reminded us the harm the gears do to Luffy's body thoroughly, and on purpose. Their time is coming to an end...
quick refresher to all of this. anybody who read the dragonball manga would know that both the kaio-ken and the super sayian form couldn't be held for long periods of time at first cause of it's strain on the body and life of the user.
the kaio-ken couldn't excede the will of the user in which they could only exerate as much as there body were willing to permit. goku felt that effect when he battled vegeta and his body was losing strengh and became useless. he also felt in during the freeza saga. even though he had learned to controll it better since he used it on earth. it still posed a threat and in the end it really couldn't do much anymore (I.E go back to the chapter when he was using kaio-ken and he still couldn't keep up with freezas speed or the episode where he hit kaio-ken 20X and bearly mangaed to inflict pain upon freeza)
point to that was that goku managed to get around the cons to this ablitiy to enhace himself and make sure he didn't destory his body completly in the processe, even though he relaized that the ablity was not enough no longer, and has since stopped using it.
the super sayian Transformation seems like just another form of god mood. but people soon forgot the cons to this ablity also. the kais and even goku himself claimed that the user couldn't stay in supersayin form forever because the strain would take a toll on the users life. he mentions to trunks the first time they meet that at frist it was hard to just transform into the super sayin form and maintain but got easier over time. elder kai even mentioned to goku that fusing while in super sayin mood was risky simply because he might never be able to power down, and would prove dangerous as the users life will start to feel the effects of the strain of maintain it's power. this form while it seems like is the dragonballs cheat code for the heros to win there fight, a few key points are always missed: Gohan techinicly wasn't in supersayin form when he fought super buu and goku had to beat buu with another method rather then the super sayin 3 form**(which took unbearable amounts of energy and staminia just to maintain the form, not to mention the strain it cause him when he was alive)**even the mighty super sayain form has had it's Cons
with that being said we can now move over to the gears that luffy use since it has been likened to the kaio ken and what not. luffys new moves are taking tolls on his body, this we can all see. his friends and enemies are taking notice to this also and have growing concern over luffys useage of the ablities. luffy wouldn't wish to throw his life away simply due to the fact that he wants to become pirate king. while there are major pros and cons to the gears, i highly doubt that oda will just dump the ideas away. knowing luffy he will probably find a way to turn the cons into pros by some means later on in the series. all the ideas for the major players have stayed with the strawhats so far**(nami still uses the new and improved tempo with dial, ussop uses his new kubuto slingshot, saji has the diablo leg attacks, zoro the ashura style attacks and chopper, though i doubt he'll use it over and over again, has used the monster form of his again)** while they don't cause the user to much damage to themselves, all of there new ablities are what got them past CP9, thiller bark, and defeated a few of the PX models. luffy will undoubtly be the stronger of the strawhats, that much can be said. And he knows that the gears, while life threating, are his greatest assats so far. he also mentioned after his fight with bruno that his body has not gotten use to the gears yet, indicating that there probably is a way for him to get past the risk sooner or later.
the most likely of choices will be the use of Haki that people are starting to assume. im willing to bet that before this arc ends luffy will still be using a form of his gears, but will intergate something a little less life threating or something that will cease the risk such as haki or ambition or whatever they call it.
i don't expect oda to just get rid of the gears, i believe he'll hold off of it for a while but never get rid of em fully. there the only thing so far that gives luffy the ablity to use soru and his incredible strengh