considering most (99% like) of the linux installers AND games work for me (KDE neon distro, which is based on Ubuntu 16.04 LTS), I have to thank you for your work a lot.
It would be a bit less annoying to have "not possible" (by whatever undisclosed reasons) linux support marked directly on the game card, so one can know when you already tried and failed to provide the linux binary (I mean especially with games, which do have linux versions elsewhere), just by checking the game page in store. Also marking in-dev support for games where binary is expected. But I understand this is probably a lot to ask, so I don't expect it to happen, just would be nice. Or at least some forum thread with list of games where there's no expectation of linux binary in near future.
About the linux client for galaxy... not that I'm really in a need of one, but one open-sourced-highly-modular-low-resources-hog, which would work as background downloader/updater, providing also cloud saves and similar simple to program things would be nice. And I don't think there's too much of work needed from GOG, if you would let community on it, by providing detailed specifications and API and some git server, then you would probably get some pull requests of reasonable code over few months/years. Maybe eventually written in portable-enough way, that you would be able to reuse that even in windows client. There are already people here who know your current API inside-out as much as the sparse info (and/or RE) allowed them. There are certainly people here who can write portable efficient C++. Possible problem is, that it would take to see serious dedication on GOG side to invest a spare time into contributing for commercial subject. But first the business decision to take such route would be needed.
About Witcher 3 linux port - while you are not directly responsible, this shame (both the result, and the communication fiasco) falls also on both, as the pre-order communication was almost a fraud.
I can see how in the start and end this problem originates at players side - would they buy only games with linux binaries provided, without using wine/PlayOnLinux/etc in every other case, the market numbers would be less skewed toward windows platform. Also it would be easier to see how minor the linux really is (probably too minor, so in the end we get treated by the way what we pay for or even extra over it, as a group). I just hope it will be more diverse over time. But sometimes this business way of silence is not good, even stating the obvious "we can't disclose details" is better than total silence.