In '05 minus factory options.
Base - Single tipped exhaust, cloth interior, no sunroof, single disk CD player, OnStar, majority came with alloy rims. Most didn't have ABS or TCS.
GT - Dual tipped exhaust, leather interior, sunroof, 6 disk CD player w/ XM, Onstar, different wheel combinations. Came with ABS and I believe TCS.
GTP - Majority the same as GT, but supercharged.
GTP Comp G - Same as GTP with tapshift transmission and few other loaded goodies. Along with HUD and so on.
GXP - V8 5.3L LS4 front wheel drive flop. Leather all the other loaded jazz.