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Your phone has sensors in it to shut it down if the temp get too hot. Must be doing a number on your battery if it is getting really hot.

Games are CPU intensive and heat is a by product of same.
 
The CPU things and your hot hands as you play! The GPS may be because it's in your car and the sun is coming through the windscreen.
Just take a break every now and then, and instead of putting your phone in your pocket, just leave it on the desk to cool.
 
I'd only be concerned if it gets hot doing normal tasks like wifi browsing, email, twitter, pages etc.

I just had a 5s replaced that heated up while doing non cpu-intensive tasks, new one is much improved.
 
How hot is "HOT"? As a long time Quad Core android user, I imagined a dual core 1.3GHz won't get that much hot. I still haven't used it though so I can't tell.
 
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