While connecting to the server, required content is downloaded from you, which is set by the server developers. So these addons remain on your disk and take up space until the last one. At the same time, these addons do not appear in subscriptions.
These addons also include those that are downloaded dynamically while playing on the server

Now let's move on to the most important thing: Removing these addons

Close Garry's Mod first, if it's open

Then open the game folder and go to the folder garrysmod
If you opened everything correctly, you should have the following folders in front of you

Delete the following folders: download and cache

Completely close Steam

Now navigate to the steamapps folder on the same drive. You can do it the same way as shown in the screenshot.

Navigate to the following path: steamapps/workshop

Delete the file appworkshop_4000.acf

Go to the folder content

Delete folder 4000

Log into Steam and then into the game. Wait until your addons are installed and that's it - cleaning is over!

If you error downloading your addons and they are highlighted in red, just unsubscribe and resubscribe. It should help.

If it didn't help, then you did the cleaning in the wrong order or missed some step. Try restarting game or Steam. As a last resort, if nothing helps at all, try clearing the download cache in the Steam settings