Have you tried clearing your local cache from gmod? Sometimes a corrupted file or virused file will cause a game crash. Also have you checked any extra mods/other games linked to Gmod such as CS:S and HL2:DM for errors/corruption? Could be that a file is corrupt or damaged.
The best thing to try first with games on steam is to check that your local files for the game are not corrupt. Right click on them in your library, Click properties, click on the local files tab and then click "Verify Integrity of Game Cache". That pretty much sorts all of my issues with Steam games.