If the game makes use of GOG Galaxy for anything, there probably wont be a Linux release here at all, as GOG has no Galaxy client or RTL for Linux at the moment so developers relying on Galaxy don't release their Linux ports here for the most part as they'd have to make a custom non-Galaxy build which is too much extra work.
It might be worth seeing if the developers have mentioned any plans to support the game on Linux on GOG, or asking them directly before buying it here.
Hopefully GOG will be able to hire more developers soon with the influx of cash from Gwent's popularity and some day be able to prioritize Galaxy for Linux higher though.
There are non-Galaxy builds for every single game on the site.
My point is that games that exist on GOG which have Linux versions that do not exist on GOG, and the reason why the Linux version is not on GOG as stated by the developer of those games, is that GOG does not have Galaxy available for Linux.
This is of course the decision of the developer to not divide their limited finite resources on supporting two different builds of their game, one with Galaxy support on WIndows and one without Galaxy support for Linux on GOG. Regardless of why it happens however, the fact remains that it does happen and there are countless games in the GOG catalogue which have Linux builds on Steam but not on GOG due to the reason that I cited. It is therefore quite reasonable to assume that other developers are likely to make the same decision since it appears to be the common case that if a game developer decides to implement Galaxy features and services into their game when they release it on GOG.com, that they are quite unlikely to release a non-Galaxy featured release for the Linux platform here on GOG, regardless of whether it is theoretically possible for them to do so.
It is a business decision in the end, and it does in fact happen.
And if you're referring to "Galaxy builds" as "downloadable installers that do not include the Galaxy client", then we're talking about 2 completely different things. I'm talking about a game that is available for Windows on GOG.com which provides Galaxy features such as achievmeents, multiplayer matchmaking etc., but does not ship a Linux build at all on GOG.com due to GOG client and libraries not being available for the Linux platform at this point in time, while they ship both Windows and Linux builds of their game on Steam etc.
Hope that clarifies things if there was any confusion.
Update: I just checked and this game does not have Linux platform support on Steam. The chances of them shipping a Linux build on GOG but not on Steam is quite slim, so I would presume that there is no Linux version of this game at the present moment at all. So if anyone is wondering about Linux support, there's the answer.