UPDATE, 12th of October: In a curious turn of events, CD Projekt RED have opted to release Thronebreaker: The Witcher Tales on Steam after all. Yeah, we're not sure what that 'GOG EXCLUSIVE' tag was all about, either.
In any case, should you choose to get the game on Steam, you'll get a number of additional goodies as well, such as a copy of the very first Witcher game, official soundtrack and concept art, a digital copy of the Gwent Artbook by Dark Horse, and the Fox Children graphic novel by Dark Horse. Enjoy!
As it turns out, CD Projekt RED have decided against releasing Thronebreaker on Steam, and Gwent's standalone singleplayer offshoot will now be released as a GoG exclusive. Unless 'PC version ONLY on GOG.COM' reads otherwise in some odd way?
Up until now, we've kind of expected for Thronebreaker: The Witcher Tales to follow the usual CD Projekt RED release schedule, where the game would simultaneously come out on every major video game marketplace, or that perhaps they'd stagger the game's release on other services a bit.
This has now been proven wrong, however, with the launch of Thronebreaker: The Witcher Tales' pre-order website, which very obviously points out that the game will be an exclusive GoG release.
It is a bit early to say whether CD Projekt RED are pulling a Bethesda or an EA on us, and if this sort of approach will affect whether Cyberpunk 2077 won't be coming to Steam, but it does stand to reason that they could conceivably do something of the type with the amount of hype that their upcoming FPS/RPG has garnered so far.