I'm no mod author, but I do see the advantage to create with Steam Workshop. But as a GoG customer, it sucks.
I don't mind manually handling mods, have done it with Skyrim since day 1.
Why can't Paradox make a ingame "workshop" that lets you log on to Steam Workshop with your Paradox account, from within the game? Using the Overlay from Gog Galaxy client?
I bet Steam will do everything in their power to "force" every gamer that loves modding, to use Steam platform...
Or Paradox could take a stand... prevent modding on Steam Workshop, only use Paradox Modding, get some cash-back to the authors and let the community flourish.
This is just sad. They advertised the game "with Mod-support" and didn't mention it was Steam only that has mod support.
Other platforms will always get stuck in the shadow of Steam... No real and fare competition.