How do I load all addons, even if I am not subscribed to it (but have it downloaded already)?

In Garry's Mod, there are addons; some of which are legacy (.zip files that you can download off garrysmod.com) or workshop files (anything with .gma).

I've downloaded some addons, most of which are from the workshop. On my main account, all the addons work fine and they work as per normal... However, when I switch over to my alternate (on the same computer, but different account), all my downloaded addons seems to have stopped working.

Is there any way to force GMod to load all the addons in the garrysmod/garrysmod/addons/... folder? Even if you didn't subscribe to it?
... Or subscribe automatically to them all?

The unsubscribed addons will load as long as you're in offline mode on Steam. When you're in offline mode, Steam does this wonderful thing it should always do, and forgets which addons you are subscribed to, or not; and loads every single file you have in your addons folder.

WARNING: if you have 200+ addons with a lot of custom content, you will want to remove some addons before you try and launch in offline mode because like I said Gmod will load everything you've ever subscribed or unsubscribed to and haven't physically deleted from the addons folder yourself.