In a case like yours ..first I would delete any and all reference to the GT3 cars ..so check your packages folder and start the game up (launcher) and click on the box ..you will now be presented with EVERYTHING you have installed. Click the "refresh list" highlight any GT3 car and delete them (use the delete button) ..remove EVERY 397 GT3 car. Close down the launcher ...now run the launcher again and it now should install all the GT3 cars...keep reading IF it does not ... If the above fails? then I would delete the game via steam ...first copy your packages folder and place it somewhere (steam does not delete your packages folder, but better to be safe) now delete your packages folder yourself, (it will now force steam to install a fresh packages folder) when you ask steam to re-install, it will also install all your subscribed workshop items Make sure everything is working before doing anything reference the packages folder ...now you can start adding files from your old packages folder to the new one.