Try and validate your GMod files, go to your library (The place where it shows all your games), and right click garrys mod, click garrys mod, and search for the button that says "Validate integrity of game cache" Or something like that. Should fix any missing or corrupt files that steam might run into, always fixes my problem when I cant get a game to work, Also, try reinstalling the game.
If that doesn't work, we're going to need more specific details.
You'd be amazed what validating your files will do.
