mqstout: This is precisely why GOG Galaxy's multiplayer layer should have been designed properly: without player identification and authentication, and with LAN/direct-connect fallback baked right into the client (VPN-in-Galaxy basically).
Instead, GOG chose the DRMful route.
The moment you use galaxy you already have identification and authentification, Galaxy is a browser and a network for your account, it won't work without, no matter what happens behind that, you are already identified.
LAN direct connect means NOT using any client or gaming network infrastructure for communication
mqstout: becomes a cookie in your browser
So it's identification.
Using different accounts accomplishes exactly the same thing as getting a new cookie with the difference that you can easily switch back to the other account.
It's not about GOG, it's about the games. The programmers have to write LAN code, not GOG.