Try the support forum for gt6 at playstationnetwork.
but maybe this can work.
Do you remember what version of the game you had before internet?
if you still have your savegame, back it up to usbstick.
then remove gamedata (not savegame!!)
1.--disable internet on your ps3 and start game.See if the savegame is accepted.
2.--exit game, then enable internet, go to the gt6 game icon and press triangle, select "search update"
download first update, after install into ps3, disable internet, start up game
2.1.--If your old save is accepted? , save game (press start and select save).
if not, exit game and copy savegame from usb back to ps3
4.--exit game, then enable internet, go to the gt6 game icon and press triangle, select "search update"
download next update, after install into ps3, disable internet, start up game.
5.--repeat part 2.1
6.--go to part 4 and repeat it for all updates
At one point the game could recognise your savegame, and while you startup after each update, the savegame could get updated.
warning, this could take you hours... even a day or two.
I have an SSD in my ps3 and 100mbps internet and it took me 18 hours when i switched from disk version to digital version.
So good luck.