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This is the kind of thing I will consider heavily not buying here and looking at Steam. Time and time again I have purchased these types of games (Heavy DLC, Mods, multiplayer) and been disappointed when compared to communities elsewhere. No, this isn't GOGs fault per say, but I am tired of not getting updates in a timely fashion and being unable to engage in MP due to complete lack of local community or crossplay. Kills me. May not be the case here of course, but I will not be fooled again to simply purchasing on GOG immediately.
This is a slitherine game that does not depend on steam mod workshop support. Your choices are gog or Slitherine directly, with slitherine direct giving slitherine a bigger cut.

As for multiplayer, remember that the DRM free except for serial Slitherine version is first class citizen, not Steamers. So there shouldn't be a multiplayer mismatch from other versions since both Slitherine and GOG are the drm free versions. This is a serious wargaming company so they don't do the shenanigans, like in Spellforce soul harvest with more than a week multiplayer mismatch. Wargamers and grognard are a small , old fashioned group and annoying them is death for business since wargamer audience is not easily replaced.

I'd say buy it here or on Slitherine. I prefer Slitherine to feed them money, but convenience of GOG is better.