Try deleting your cache. Go to steam>Library and right click on GMod, than, go to the properties tab and go to the local files. Next, go into the folder name garrysmod, and go into the folder called cache. Dont delete teh cache folder, but delete the files inside of it. It wont damage your game, you will just need to re download the Lua files and maybe the maps if you dont have them subscribed to your workshop. For me it helped, as well with some crashing.