Last few days I have been smelling gas after I park my car, shut it off and get out. Even if the car has been sitting as much as an hour or more after I park it. I have also noticed that I have been getting pretty crappy gas milage and when I idle in gear the rpm variate from 500rpm to 700rpm(mainly closer to 500rpm)Could somebody tell me what I have to check or do or test?

Ahhhh.. well, I'll tell you my gas leak problem, maybe it is yours as well. My 1988 GTA has a leak, the tank is not rusted, heck it looks mint, but the problem is that the steel filler tube had loosened off the tank a tad, allowing vapours and small amounts of fuel to seep out.

Do you see any fuel on the ground after a few hours of it sitting after running? Can you localize the strength of the fuel fumes?

Actually man the gas fumes seem to be coming from under the hood. I can sit in the driver seat and smell it sometimes and when I get out of the car (driver's side) I can defently smell it while walking towards the front of the car........

Hmm.. well, then you may have a leak up front. Here is my suggestion, take some vise-grips and gently block off the return line (rubber line) at the front of the engine. this will the pressure will stay in the system instead of returning to the tank, you can trhen see if you can pin point a fuel leak somewhere in the engine bay

Check your fuel lines, injector o-rings, etc. You might also have a problem somewhere in the emissions crap.