There are multiple ways to clean your garrysmod, we shall cover three of them here.
1. Delete almost everything

Go to "C:\Program Files\Steam\steamapps\USERNAME\" ("C:\Program Files (x86)\Steam\steamapps\USERNAME\" on 64-bit; Or where ever you installed steam.) and delete the "\garrysmod\" folder. Note: This will NOT require you to re-download the game; Steam will extract the necessary folders when you next run Garrysmod.
2. Delete fucking everything

Simply go into your steam games list, right click "Garry's Mod", and click "Delete local content..." , then click OK. Then go to "C:\Program Files\Steam\steamapps\USERNAME\" ("C:\Program Files (x86)\Steam\steamapps\USERNAME\" on 64-bit; Or where ever you installed steam.) and delete the "\garrysmod\" folder. Note: This will require you to re-download the game.
3. Delete unnecessary things

Go to "C:\Program Files\Steam\steamapps\USERNAME\" ("C:\Program Files (x86)\Steam\steamapps\USERNAME\" on 64-bit; Or where ever you installed steam.), Then go into the "\garrysmod\" folder, then go into the "\garrysmod\" folder inside the first one, and delete all the folders and files except:

1. models
2. materials
3. particles
4. screenshots
5. sound
6. shaders

If you wish to keep your addons, don't delete the "\addons\" folder. This will delete your settings, including any custom key-bindings("\cfg\config.cfg"), tool presets("\settings\presets\"), addon data ("\data\"), and gamemodes("\gamemodes\") you have; Don't delete those folders if you want to keep any of those. If you don't want to re-generate your spawn icons, don't delete the "\spawnicons\" folder.