Too.good: Please inform yourself more. You own nothing neither on steam nor gog. What you buy is a license under it's terms you can USE the software. Online service is still required in manor lords look it up on wiki it's literally called epic online services.
Agreed with OP GOG needs to implement mod support in it's client maybe this would help to increase Galaxys popularity.
"The Court of Justice of the European Union declared it legal for consumers to resell previously bought and played downloadable games and software. This decision arises from a legal battle between software reseller UsedSoft and developer Oracle in German courts."
https://game8.co/articles/latest/steam-gog-and-others-must-allow-reselling-of-downloaded-games-in-eu-Pelihemmo-: I always want to support GoG, but this game will be modded sky high. Steam Workshop support is very likely. Fortunately many mods will be found from Nexus and other modding sites... but not all. It depends on the modder where they publish, and the fact is many who publish in Workshop, don't do it elesewhere.
I wish GoG would have mod support. It needs to happen for GoG to grow.
IggyDaDino: I just use Nexus. It's really a problem with Steam's ecosystem and I really dislike supporting what they've done.
It is possible to get the files from workshop and as long as the game is the same install it yourself. But yeah, a lot of steps added.
How do you download Steam mods? I would walk this extra mile if it worked. Some years back I tried it, you had to do it via another web site, but it did not work.