So to summarize:
- You can play LAN on both GOG/Steam versions
- There is crossplay between GOG and Steam users
- You need GOG Galaxy if you want to use the Online Multiplayer feature in the GOG release
- GOG Version updates lag behind Steam updates

So basically you have to get the "optional" client if you want to use the game's full feature set with a GOG version?

Because.
Care to give a howto?

Well, I meant that it is not required that Galaxy is running when you play mp.

Maybe there is such an opportunity, but I do not know about it. By default, the game creates your lan profile based on account data from steam/Galaxy.

Sorry for misleading.