What I’m saying is that Steam has the same problem as GoG - no guarantee that other players are available or will want to play multiplayer. It’s just that Steam is bigger than GoG and has been around longer, so that’s not as likely; but not a guarantee.
My understanding is that there is a fair amount of multi-player GD going on in Steam. I don’t play multi-player.
Based on Zantai’s post it doesn’t look like there is anything for the GD devs to do, and that it is up to GoG to make Crossplay happen. I could have misunderstood his statement though.
It sounds like you should contact GoG’s customer support and request that Crossplay be enabled for Grim Dawn, or ask if they have plans to do so.