How can I prevent a new computer from downloading my "subscribed" add ons in Garry's Mod?

1. 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".

This didn't end up working for me, it stays unchecked but initiates installing the addons on game startup anyway. 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.

2. Another option is to try to backup your addons from your desktop, then unsubscribe from all of them through Steam Community. 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. You could try backing them up from your "garrysmod" directory and a file called "appworkshop_4000.acf" located in: steamapps->workshop.

Working with "appworkshop_4000.acf", you could also try to set all of the "NeedsDownload" instances to "0". 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. I tried deleting all the info from this file too but because of it takes lower priority, it gets "refilled" instantly.

*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(){jQuery('.btn_grey_black').map(function(){this.click()})})() 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 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..

Edit: You can also have a look at this: http://forums.steampowered.com/forums/showthread.php?t=2470784

I will try it myself and try to summarize it to when I get the time

Here is a strange little trick I learnt and taught my friends: note that, however, I have only tried it on Mac. It may still work on other platforms, but you will need to be quick (Elaboration below). 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)

Here it is: With your Gmod page in the library open on Steam, click on the "Play" button. Now, VERY quickly, go to your network connection tab (AKA for Macs, the Wi-Fi bars at the top) and turn your internet off. When the game has fully loaded to the game title screen, you can turn your internet back on.

Not quite sure why it works, maybe the game checks the Steam servers during loading to look for data such as subscriptions?

Please note that to play online on a new computer, you do have to start the game online first. Hopefully it should still work with the above method, but if for some reason it does not, just open the game normally, then quit it when it opens and procede as above.