I had somebody IM me who was unsure about the actual process of deleting the game files, so in case anybody is wondering exactly how to do it, here is my response to their IM.
"From the main PS3 startup screen go to the "Game" icon, then up to "Game Date Utility", then "Gran Turismo 6", press triangle and press X on delete. That will delete the GT6 installed files. Make sure you go into "Game Data Utility" and not "Saved Data Utility", if you delete GT6 under "Saved Data Utility" you will end up deleting your game save and if you delete your game save you're gonna have a bad day."
Cheers