I have many reasons to believe Luffy's usage of Gears Second and Third is coming to an end.
Allow me to explain my logic. Since the first time he used the gears, we were able to see how Luffy had to withstand the severe pain and suffering both gears caused on him.
In this link, we can see him fighting Blueno, after his first time using Gear Second. He states he's "dead tired":
The next time he uses his gears are when fighting against Rob Lucci. Do notice that he makes his best to try fighting him without any gears, at first, even though Luffy knew how strong he was, and knowing Robin was in danger and he had very little time to catch up.
However, he then uses Gear Second as the "last resource", a desperate move which was completely needed, in order to be able to hold Lucci and get Franky out of the room to save Robin.
In the link below, you may see that Lucci notices Luffy is out of breath after using it. His body was growing tired incredibly fast.
During the fight, Rob Lucci finally understands how exactly Gear Second "works". In the link below, you can see his explanation of it.
Notice how he bold the phrase where he implies Luffy is shortening his own life with this method. Luffy, as we know, would not care to risk his life to the extreme if he's fighting for his Nakama and his dream. However, even that has a limit. Later in this fight he is forced to let go of Gear Second, and his power is severely diminished, so much he almost dies. In the end, he's able, with his strong will, to re-activate Gear Second, even at the blink of death, and use a final move which defeat Lucci, before falling down and not being able to move.
After Enies Lobby, they are able to get to Florian Triangle, and find Thriller Bark. At the end of the arc, they are finally able to beat Moria. Before the fight against Kuma begins, they have a little time to talk. Luffy is completely down, and wont wake up for a good time. Remember he used Gear Second this fight as well, along with the 100 shadows, which certainly contributed to weakening his body even more. However, it was only after his usage of Gear Second, that he finally plumged into unconciousness.
At that time, Usopp looks Luffy and states something very, very important. Please look the link below.
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...