Now that these people are speaking about "games as a service", I'm becoming nostalgic of the good old concept of "game as a game"...

Galaxy is DRM in the way Steam is, meaning sofar only a few (if not just 1) require it to operate. The reason its required is because Gwent needs an account to work, if you ever try it you'll understand. it's almost like an MMO then a multiplayer game :)

Galaxy provides:

Cloud back-ups (Authentication)
Forum access (Authentication)
Library access (Authentication)
On demand game installs (Authentication)
Auto Patching (Authentication)
Chating (Authentication)
Multiplayer (Authentication)
Cross Platform play (Authentication)
Downlaod of all offline goodies and installers (Authentication)
Direct access to all of GOG's web based services (Authentication)
FPS overlay (could be offline)
Game play time (could be offline)
Screenshots (could be offline)

That said its 100% optional and not needed at all, except for Gwent, as core features to the game need Galaxy... There is no way i can see the game working without it (play it and check it out to understand)

No need to quote me on these points. I have both Galaxy and Gwent installed, so I'm well familiar with it.

I hope see, that you contradict yourself in one sentence?! Either something is optional or not. And if I need something (in this case Galaxy) for 1 or more things, it is no longer optional but mandatory!
Besides Gwent, Galaxy is needed for multiplayer, so not optional again. But I agree with you that Galaxy is DRM like Steam. There is not difference to me.