Delete your C:\Program Files\Valve\Steam\Steamapps\<username>\garrysmod\garrysmod folder, load up Garry's Mod so it can create a new one, and just reinstall all your addons.
Well, when I need to, I do that. But before deleting anything manually I would recommend going onto steam, right-click GMod and click 'Delete Local Content'. It may not even be neccessary, but I do it nonetheless.
Before deleting your garry mod,because you have a probleme try doing this.Go in steam.Right click garry mod,"properties",Than click locals files,than click "verify the intigrity of this game".If it does not repair your probleme re install using tox method.
Yep, first use the method of what Eddie said, I was about to say the same thing. If that does nothing, you will just have to manually un-install it as Tox said.
Rename the garrysmod folder to gmodold or whatever. The main one, not the garrysmod folder inside it. Now try and run the game. It won't run just yet. Instead it will create a new garrysmod folder with all the vanilla stuff. When it's done it will run. Btw, I just reinstalled GMod like 2 weeks ago so I know this works.
if you REALLY have issues unsloved by reinstalling then delete the GCF file - they are usally a few GB each so last resort here