Gran Turismo by the Numbers: Drivers guide database, several forums and a Gran Turismo FAQ.
Gran Turismo Paradise: This site has everything on Gran Turimso, Gran Turismo 2 and Gran Turismo 2000. I am also giving away a free game.
GranTurismoX.net: Game information and forums.
GTA: The Gran Turismo Authority: Includes information on cars, tracks, updates and contests for players.
GTPlanet.net: Includes car of the week, photo of the day, and information.
The Unofficial Gran Turismo 2000: Information and screenshots.