This happened to me, and it was back when the game couldnt run on the computer I was on.
That's not your case though but try to uh...
1. Go to library
2. Go under games tab
3. right click garrys mod
4. properties
5. go under local files tab
6. Verify integrity of game cache.
This usually fixes my gmod errors because I poke around the folders frequently.
Also while ur in properties, under "local files" tab, do a defragment cache files.
That helped me too.