Foclock.215: Yeah, that does sound fun! I'm glad you see it my way.
Here's are some interesting followups to the idea...
Obviously big name AAA games that are available only on Steam/Uplay/Origin are pretty unlikely (Call of Duty series, Tom Clancy newer titles, ArmA 3, Valve games, Battlefield series, etc.), but there may be some AA, A, or other games that stand a possibility to show up sometime, so the question is...
What existing FPS games on GOG right now would people like to see gain GOG Galaxy multiplayer support?
What FPS games that are not currently available on GOG but
which might realistically stand a remote chance of being added to the GOG catalogue sometime in the near future would people like to see here?