actually scratch that I looked it up in the wiki and apparently Rob Lucci tells him (I took this straight out of the wiki by the way).
"Due to this, as also pointed out by Rob Lucci, Gear Second hacks away at Luffy's life force dramatically, increasing a chance of having a much shorter lifespan because he is starving his body, though Luffy rebukes it, saying that it does not matter as long as he saves his friends."
It explains how and why he is starving his body for nutrients but I won't post all of that, so its under the Gomu Gomu no Mi/Gear second.
Ah man...i don't want luffy to die. The guy is the most epic character ever.
There are two instances exactly where it is voiced that the Gears reduce Luffy's life: Enies Lobby by Rob Lucci (who puts it very bluntly) and on Thriller Bark by Usopp (though it is more ambiguous).
Since Luffy does seem to have incredible health he might not fall ill like Roger did (and if he does he has Chopper as it has already been pointed out).
But I once heard during a discourse about health in connection to the life force of a person, that our lives are to be seen like batteries: Our bodies do contain a certain amount of energy/ life force and once it is severely depleted there is no simple turning back and recharging it like we do with a battery.
That is why some people who suffered serious physical as well as mental ailments may show signs to have aged many years in a very short amount of time.
So it might be that Luffy will not fall ill but could literally drop dead from exhaustion.
But I actually want him to live a long life full of adventure.
i still dont get why luffy can use gears and why is the only that can
Whenever Luffy engages his "Gear 2" he goes in a horse stance, where he pushes on his knees and by extension on his lower legs, making it basically work like a pump.
By doing so, he stimulates his blood to flow much faster through his body, thus raising his blood pressure to levels where usually people's ateries would burst - the reason why Luffy is the only one who will ever be able to use it.
For a short amount of time it will work like a stimulant or drug enhancing his physical strength and endurance and numbing pain to some extent.
The immediate after-effects though are exhaustion, the onset of the pain that was numbed (amplified by his body's exhaustion) and hyperthermia (the reason why Luffy's body steams). The long-term effects of the Gears could be stroke, heart failure or kidney failure to name a few...
Hope this helped!