According to GamersMint.com, a user who had ample proof that he owned a retail copy of Gran Turismo 5 has revealed that the game will install data worth 6.4GB on your PS3 HDD; and it is said to take 40 minutes to install. However, one thing to note is that it is an optional install, which is probably good news. Another interesting thing to take note of is that the game will “decompress and install” data during your play of the game.
And for all you Kevin Butler fans – it seems that Gran Turismo 5 is what it is today ( with over 1000 cars, and realistic damage, and day/night cycles). Here’s the video about Kevin Butler’s feat: