You could also try temporarily disabling any antivirus software you might have installed, as they may conflict with the game. If uninstalling is not an option, temporarily disable or exit the software before starting playing. It is advised to uninstall those programs or disable their Overlay functionality and try to launch the game again if you are having start up problems, such as black screen on start up. RivaTuner Statistics Server (aka MSI Afterburner).Any software that may contain Overlay functionality (mostly Voice Chat applications).

There are a few programs that may interfere with Garry's Mod and other Source Engine games, such as:

Uninstall or exit programs that are known to cause problems If you have Team Fortress 2 installed, you should complete steps above for it, and any other installed Source Engine game as well. Lately Team Fortress 2 started to cause problems with Garry's Mod.

You can learn more about game cache verification here Verify Team Fortress 2 and other mounted games, if any

Press 'Verify Integrity Of Game Cache' button.

Select 'Properties' from the dropdown menu.

If that doesn't work for some reason, you can do it manually: Sometimes game files might get corrupted and a file verification is necessary. If when you start Garry's Mod the screen is black - your display settings might be wrong.

How to make gmod addons