At present Galaxy has no means to sign into playfab, we are currently working with GOG to get this up and running. LAN/Direct IP games do work between Steam and GOG however.
The GOG version of the recently released Quake II: Enhanced Edition also uses the Krex engine and Microsoft's PlayFab network for multiplayer, and it supports crossplay between GOG, Steam, Microsoft Store/XBOX app, and Epic Games versions of the game in the same hosted sessions/lobbies.
My advice would be to use the Galaxy API to get the user's GOG ID and then use that ID as a custom ID to create an account automatically on PlayFab. See https://learn.microsoft.com/en-us/rest/api/playfab/client/authentication/login-with-custom-id?view=playfab-rest
But, you should really ditch PlayFab and be using Epic's online network (EOS) instead. It's the best cross-play solution, and unlike PlayFab, it is free at any scale. PlayFab is only free for development testing from what I have read.