Before continuing you will need to exit steam entirely first, it also must not be open in task manager at all. maybe open this link in a browser if you dont use the browser version of steam,

Locate the correct folder you will need:
C:\Program Files (x86)\Steam\steamapps\workshop\downloads
The path may be slightly Different for you, But i'm sure you'll be able to figure out how to get to it.
When you get there the folder should look like this, But with files inside.


Delete all the folders and files in the previously mentioned Folder path, then relaunch steam.


Goto library > Downloads, and press the update button on gmod if its there. (this may not be needed but it fixed it for me)

Now after all that, relaunch garry's mod after its done downloading If you pressed the update button. The addons should now work and be in the normal area they appear!