First off, we are going to need to get to our main Garry's Mod directory. It's usually located at:
C:\Program Files (x86)\Steam\Steamapps\Common\GarrysMod\garrysmod\. It's simple, and the easiest part of the guide.

In your GarrysMod/garrysmod/ directory, you will see a folder named "cache". Delete it. It usually holds up to 10 or more megabytes. Not a big amount, but we are getting to the big amounts.

Anything in the download folder are things you download when a server forces you to. Usually full of sounds, materials, and models, all raw.

Anything in the downloads folder are things servers have forced you to download on the workshop. Full of .gma files, and there should only be GMA files in there. [A GMA file is a compressed file, uploaded directly to the workshop by modders/scripters/developers/modelers/etc.

Feel free to delete the folders themselves, but I just delete anything in them.

Delete anything in the following folders:

• Demos
  
• Saves
  
• Screenshots
  
• Videos
  
• Dupes

Only delete things inside of the folders, not the folders themselves.

Congratulations! Empty your recycle bin and experience pure profit!
I have gotten 100 GB worth of space deleted by using this method. You'd be suprised how much Garry's Mod downloads and how much space it takes up until now.