Yep, until Steam and GoG decide to make co gaming available you are out of luck. Its always a problem for any game that has either mod support or MP so choose your purchases carefully.

You cam look on Nexus but not sure it has all the mods from Steam there.

The authors would have to load it up on w/e site that isnt affiliated with Steam.

Well, I went with GOG for the full offline capability. Looks like I will need to buy it when it is on sale for cheap on STEAM too :(

https://greasyfork.org/en/scripts/29538-enhanced-steam-workshop-downloader

Something like this work? No guarantees or warranties offered.

Although if the GOG executable wasn't updated to read from mod folders, you're gonna have a bad time.