Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General D:OS2 on GOG and Steam Workshop content.

I don't know if linking external websites is allowed in the forums, but... I think GOG players can just use this: http://steamworkshopdownloader.com/ (Steam Workshop Downloader) to manually sync their mods with Steam players.

I've used it to install Workshop Skyrim mods via Nexus Mod Manager and it works like a charm. Problem solved, I guess?

Nonetheless, I'm still deciding where to buy the game. I'll probably get it on GOG, but that's just because of personal preference in their business policies.