If you go to the GTA Source Page web site it will tell you everything you need to know about the differences:
http://www.gtasourcepage.com/
But the diferences are few like interior trim, wheels, WS6 suspension, stereo system, and other creature comforts. Like someone said a more luxury packaged Trans Am but with lots of handling. The engines are the same as regular Trans Ams but the rear end might be different depending on options. To be sure it is a GTA and not one with fake badges look at the sticker in the console and if you have the Y84 code then it's a real GTA.
Good luck, gotta go to work,
-Ozzy