
If you're willing to go the extra mile, and desperate for a friends-only game between the two services, you can try this.

You can create a friends-only game, but then you can only invite your friends in the service you are using (Gog, Steam), because they need to be informed of the "Galaxy Lobby ID" (that an invite contains).

Problem is, in my experience, there is no way to set a password, so unless you have a group of exactly 4 friends, anyone can still join the game after your friends are connected. You can either play an open game, which anyone can join using any client, including your friends, after they find the game in the main server list. The issue is the friend lists, which are not synced in any way between Gog and Steam. SW2 supports crossplay between any Steam/Gog version of the game.

No, you can't invite friends from the opposing service (GOG/Steam) to a friends-only game. TLDR: Yes, crossplay is supported, via open server (that anyone can join, max 4 players).
