Remove all addons. Don’t necessarily delete them, just move them into another folder inside addons called ‘archive’ or something. If this solves your problem, then one by one move each addon back until the problem occurs.
In Steam, right click the game you are having troubles with, select properties, then ‘Set Launch options’ in the box, add -sw
This will make the game start in windowed mode. If this resolves your problem, try re configuring your graphics settings
If your game loads to the main menu, select options, the keyboard tab ( should already be there) then advanced. Check “Enable Developers Console” Press the ~ key and look for any errors. If you can join a game without crashing to desktop, it is recommended you look in the developer console for errors. most likely the reason will be there
Backup your addons, and re-install garrysmod. Make sure it boots properly first before replacing the addons!
Make sure you have the following installed (not necessary, but will help fix the purple checkerboard ( missing texture) and red errors(missing models))
If you have anything to add to this, or any revisions please post a reply, thank you!