If you haven’t, you should uninstall Gmod from Steam, then go to your steamapps/[USERNAME]/garrysmod folder and delete it, just make sure you backup any custom settings files or what not before hand, then reinstall Garry’s Mod and try again. You probably had left over files from before the big 13 update causing issues.
Did you make sure you weren’t subscribed to any Workshop Addons?
You should also make sure there are no conflicting programs running and you should start up CSS, HL2, HL2EP1 and HL2EP2 so they can install the needed files that Garry’s Mod reads for you to be able to mount them in game if you haven’t already.
Yet another bump, [SUB]because I really want this fixed soon.[/SUB]
Tried those (At least C:SS and HL2EP2, as I don’t have HL2), didn’t do anything (yet again).
Trying Half Life 2 Episode One now, going to post results after I try and see if it will work.
This image may help, it shows the problem in the earlier version of Gmod. In the newer one (newer in terms of the Gmod 13 BETA that you had to get a key to use), it worked like a charm, save for the console at least.
If removing drives fixes your games, you have bigger problems than gmod. When was the last time you’ve reformatted your computer? Your registry files are probably all messed up, especially if you have “registry fixer” programs. Have you tried completely uninstalling/reinstalling steam?