It amazes me that people think this is going to be more significant than what is... the focal point is single player which has been well stated. MP will be nothing more than an optional part that can probably be safely ignored. Like the vast majority of games already on GOG.
The way they are talking it appears to be something like players being able to meet up a certain locations in-game and interact with each other playing mini games, etc.
I am more interested to know what this means for a possible Steam version? Will Cyberpunk be Galaxy-only (no Steam version), or will the Steam version require one to create and use also a Galaxy account/client, or will they really go through the trouble of creating the same online components (multiplayer, social hub, whatever) separately for the Galaxy and Steam versions?
It's possible they may skip a Steam release seeing as Witcher 3 had no problem selling very well on GOG. In my opinion I think they should if they are serious about pushing GOG/Galaxy as a real alternative platform.
If they do release a Steam version it will likely use the Galaxy crossplay feature. You don't need a GOG account or the Galaxy client if using a non GOG version. What that means is everyone, regardless of platform (be it Galaxy, Steam, console, etc) will connect to the same server which is handled by the Galaxy infrastructure on the backend which is all configured behind the scenes so to speak.
GOG has specifically been building Galaxy MP to handle this and for this purpose. The first real test for it was GWENT which anyone can play together regardless of platform (except Xbox One to PS4 players because Sony doesn't allow it). Instead it's console players connecting with PC players or other console players on the same platform.