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 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.
