Edit: This is not official but would be cool to have...
Just wanted to post that there are a few Grand Prixs in the game
Pontiac Grand Prix “Bubbleback” '86
Pontiac Grand Prix McLaren Turbo '88
Pontiac Grand Prix GTP '95
Pontiac Grand Prix SE Sedan '98
Pontiac Grand Prix GTP '01
Pontiac Grand Prix GXP '05
and here is the complete list of vehicles
Gran Turismo 5 - Ultimatecarpage.com forums