(Note: I made a post with this suggestion a while back, but new people might see this and a reminder doesn't hurt.)
It's far from guaranteed to work, and I understand life is short, but the more people who are willing to hold off buying on Steam and wait for what they want (such as a GOG release, or DRM-free installer on another store like Zoom-Platform or Fire Flower Games), the more likely a dev or publisher (especially a smaller one, who might find the money from GOG meaningful) is to release on GOG or another digital store I mentioned.
Of course, there are various big-name devs and publishers who don't care about GOG's little amount of money (compared to Steam) or dislike DRM-free. It'd certainly be nice if enough people held off buying until there was a GOG release, and I do encourage that, but I also understand the unlikeliness of changing those devs' or publishers' minds. So I understand not waiting, in some cases.
Also, I specifically mentioned single-player because online multiplayer seems to be a sticking point for GOG releases. Not many devs seem to want to spend the time porting to Galaxy, presumably again because GOG's userbase is so much smaller than Steam. (I assume shared / split-screen multiplayer is fine.)