Now, VERY quickly, go to your network connection tab (AKA for Macs, the Wi-Fi bars at the top) and turn your internet off. With your Gmod page in the library open on Steam, click on the 'Play' button. It still allows you to play online, of course, and I guess that is what you are looking for (Other wise you can always play offline) It may still work on other platforms, but you will need to be quick (Elaboration below). Here is a strange little trick I learnt and taught my friends: note that, however, I have only tried it on Mac.

I will try it myself and try to summarize it to when I get the time I haven't tried all of these things so just be careful to not delete anything you don't want to, but I hope any of this helps! - Seems like yet another thing for Steam to add to their list. *If you do end up backing up then unsubscribing from your addons, you can set to display 30 items per page, and also try this js in your browser, open Developer Tools (f12) then enter this in the Console: (function())() This unsubscribes you from all the addons on the page at once - so make sure you are filtering by Gmod and have backups of the addons (.gma's) I tried deleting all the info from this file too but because of it takes lower priority, it gets 'refilled' instantly. 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'.

How to get mods for gmod on steam