Luffy loses lifespan. It's true. It initially began when he started using the Gear system in general.
Lucci mentioned that when Luffy used Gear 2 that he was shortening his lifespan because he was pumping blood in his body at such a rate that it would destroy a normal heart, but because he has a rubber heart he is able to sustain the damage inflicted on him. However, the damage still occurs. He just doesn't get destroyed as easily by doing it.
Then, with each gear, he used more straining abilities. Each one has ultimately shortened his lifespan in different increments.
Beyond this, in each case, Luffy has been using Life Force energy. His Haki. When his Life Force energy runs out, his lifespan runs out. It's that simple.
When Luffy runs out of Haki in gear 1, 2, 3, 4, or 5 he has always had major drawbacks like falling unconscious, his body becoming "chibi", his body becoming withered, or him becoming old.
It seems that eventually if he uses these abilities too much his body will give out and destroy itself similarly to how the Impact Dial would kill someone. Like, his body itself might start cracking or something. Think Sith Lord stuff.
There was also a movie for One Piece where someone with a Vampire Devil Fruit had sucked out Luffy's lifespan and he became a weak old man on the verge of death. So, if you want an answer as to what happens when he continues to abuse his lifespan... that's it. He dies. Goes on the verge of death and then dies.
...
I say Luffy dies if he uses G5 12 more times.
