Unlike 99% of PS3 games, Gran Turismo 6 via the standard XMB (PlayStation menu).

Delete the Gran Turismo 6 entry in the Game Data Utility (XMB > Game > Game Data Utility).

The Gran Turismo 6 save system on PS3 is intentionally restrictive. While frustrating for legitimate users who want simple USB backups, the PS+ cloud method works reliably. If you are still actively playing GT6 in the modern era, —it simply isn't allowed. Protect your progress with a PS Plus subscription, and always keep a separate backup of your precious replays and photos.

This contains your primary career progress, credits, and garage. It is relatively small (measured in KBs or few MBs) and can be backed up to a USB drive.