The 6.4 GB install is completely optional. Users electing to install everything up front (rather than as they play) are reportedly looking at downtime of 40 minutes or more. Those looking burn rubber in a more timely fashion can opt to have the game install content as it's accessed and unlocked through normal play.
"If users choose not to install the data straight away it will do it in stages as new areas, tracks, etc. are accessed during normal gameplay," clarified Sony.
It's not entirely clear if this means that players who choose to install as they play will be greeted by short periods of installation downtime when new content segments are installed, or if players will actually be racing as the game installs content in the background. I'm betting it's the former.
Gran Turismo comes out exclusively for the PlayStation 3 on November 24.