Ever since i downloaded GT, my multilayer has been super laggy, my game crashed an hour ago, and since then, it wont run, it constantly crashes when i start it. Single player works fine.. I unisntalled GT, it still didnt work.. Rebooted my PC, didnt work, i even re-isntalled the game and its still not crashing.. My PC is able to run it perfectly... What can I do?

Since you reinstalled the game, it sounds like a config issue. You could try resetting the game's config files. I don't know the game well enough to tell you how to do that - but a simple google search should help.

Also, try a video driver update.

I googled it and didnt find anything (unless im blind) and my drivers are updated, i did that after my reboot yesterday