If you are experiencing your Garry's Mod freezing after launching or displaying a white screen it can often be fixed quickly with the following
1) Open your steam Library
2) Find Garry's Mod and right click on it
3) Click on "Properties"
4) Click the tab "Local Files"
5) Click "Verify integrity of game cache"
What happens now?
Steam will check your Garry's Mod game files, and find whether some may be missing or damaged.
If so, it will download the files needed for the game to re-launch.