This happened to me before. 3 things I could think of.
1. Uninstall Garrys Mod.
2. Remove all add-ons or un useful crap, like something which changes your game, like brightness levels increased.
3. Go to steam support, as there is a guide which helps a lot, including different problems and how to fix them.
Hope one of these helps.
Cya around.
- The Gamer Guy
Maybe you could run a system scan, it takes a while, but it is extremely useful