they are trim packages from the dealer.

GT1 is the base model, and GT2 is what you would think that GT is.  It makes it so people dont have to say "Oh, I have the base model".  And it helps if your trying to sell the car, you can say its a GT.  Thats what someone did to me

its more about the trim--though, it isnt as noticeable on the newer grand prix. the old one there was a gt1 without the fog lamps that looked like the se... which i hate!!