Lifthrasil: Why not? It's a perfect match for Steam (or Epic or any other forced-client store)
Also: who cares?
Ancient-Red-Dragon: "Why not" is because GOG spent many years trying to keep GWENT as a GOG exclusive to draw people to GOG and also so that they get all the money collected from it instead of giving 30% of it to Steam.
Yet now after all this time, GOG has bent the knee to Steam, so to speak, and caved in. They also did the same thing with Thronebreaker, albeit that took less time before GOG bent the knee.
"Who cares" are people who don't like to see GOG bend the knee on matters like these. GOG's lack of commitment to taking & sticking with a stand on matters like these is one of the reasons why Steam has a monopoly stranglehold over the PC games market.
The smart thing to do would have been to keep GWENT and Thronebreaker as GOG exclusives, and even more importantly, make Cyberpunk 2077 also be a GOG exclusive. That would have been a bold move which massively increases GOG's customer base and CDPR's profits.
But instead of doing the wise & bold move like that, they move in the opposite direction by capitulating to Steam on GWENT, and by giving Steam and EGS Cyberpunk 2077.
Aside from no DRM, aren't the pro-GOG-, anti-Epic-exclusivity-, anti-Steam-monoculture, anti-console-exclusives- arguments usually that the customer should be free to choose their preferred platform and ecosystem and not be forced into a specific store/system?
That kinda goes both ways.
If people want to play Gwent on Steam, Epic or Granny's Organic Digital Distribution Service, by all means let them.