I'm not sure this will work though because steam pulls your subscribed addon info before listening to this file, but you could always try. Working with 'appworkshop_4000.acf', you could also try to set all of the 'NeedsDownload' instances to '0'. You could try backing them up from your 'garrysmod' directory and a file called 'appworkshop_4000.acf'

I'm sure many people have many pages of addons, making this possibly a huge pain to do, but is the only other thing I can think of right now.

Another option is to try to backup your addons from your desktop, then unsubscribe from all of them through Steam Community.

You might need to restart your whole computer, or you might need to reinstall Gmod for it to work, I'm not sure, I'm testing it right now. This didn't end up working for me, it stays unchecked but initiates installing the addons on game startup anyway.

You can try, within steam, going to Library->Games, then Right click on Garry's Mod, go to Properties in the drop-down, navigate to the Updates tab then un-check 'Enable Steam Cloud sync for Garry's Mod'.

Steam workshop gmod noit downloading