You can find GT hanging out online with the following games:
(games in red designate an online GT clan)

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Shooters
SOCOM:Combined Assault (Playstation 2)
Army of 2 (360 & PS3)
Call Of Duty 4 (360 & PS3)
Ghost Recon Advanced Warfighter 2 (360 & PS3)
Resistance: Fall of Man (PS3)
Rainbow 6: Las Vegas (360 & PS3)

Sports Games
NBA '07 (Playstation 3)
NCAA '08 (360)
Madden '08 (360)
Tiger Woods '07 (PSP)

Action Games
Blazing Angels (Playstation 3)