The cost to run 91+ gas is maybe 2-3 bucks per tank, so really its a non-issue unless you cant afford to drive.
80,000 is pretty low for one of these cars nowadays, I would never hesitate to buy one at 100,000-110,000 miles even.
This is just from reading and listening to others, but the GTP's seem to get the exact same if not better MPG's freeway than the GT's. City driving can vary because of where you live and how you drive, but both can avg 20ish city with a mixed avg of mid to upper 20's. Reasons I say the mileage part is becuase of the different gearing, GT"s have 3.29's and GTP's 2.93 (gtp's will have lower rpms on freeway).
I'm sure you have done some research already on common problems and what to watch for when buying cars and such, but I'd figure I say a few just incase you havent heard of these ones yet.
Some common issues are having a Lower Intake Mainfold leak or leaky valve covers (recall was on vc's). Upper Intake Manifold for GT cars can crack and leak. Like any other car, wheel bearings and ti rods can become worn and loose. Most of these are fairly easy todo yourself if you are handy enough.
Some of the stuff may not have been what you wanted to hear seeing how I do own a 2000 GTP but really they are the same cars with just a few difference's. So its just food for thought.
