While Gran Turismo 5 doesn't actually require an install other than a minor 256 MB one, it evidently won't play at its peak performance that way. No, to get it running at its best, you'll need to choose to do a 10 GB install that will take you five days to get done. I made that last part up, but the 10 GB is real.
The news came straight from Polyphony Digital boss and master of all things Gran Turismo Kazunori Yamauchi on Twitter, which was then translated by andrisang. He says that the 10 GB install will make sure that the game runs "smoothly."
He doesn't really say what smoothly means (it's Twitter), but now might be a good time to free up some space if your hard drive is feeling a bit full. I think smoothness is one of those things that's important in a racing game.