purplefiction: I would advise against it. Grab it on Steam, at least the game gets support there.
blind3rdeye: To be fair, the game is supported on GOG. GOG has the latest version of the game, and all in-game features work correctly .I don't think it's fair to say that the game is not supported on GOG.
There are two things that I can think of that give the steam version an advantage:
1) Achievement tracking. As I understand it, the achievement features of Galaxy came after this game was released, so this isn't so much a matter of Steam being promised. It's just that Steam had the feature and Galaxy did not. The devs could go back and edit their achievement system to work with Galaxy; and that would be nice - but adding free features post-launch isn't something we should expect or demand. (But we can certainly request it! If you care about achievements on GOG, I reckon you should contact the devs. They aren't going to read this forum, but they will read their own feature request forum!)
2) Steam workshop is pretty good for mods; but again, Galaxy doesn't have a feature like that. Mods do work on the GOG version, but they are slightly harder to find and install.
It isn't though. Look at the forums on both and you can see which one gets attention from the developers. That is a form of support, so you know. At the very top of the Steam forum they have a master list of fixes for user issues. Where is that here? You said it yourself 'they aren't going to read this forum,' they have someone that reads the Steam forums.
If you have issues with the game, which people do despite the claim of 'all in-game features work' and when they do there is a good chance that GOG support will be telling them to contact Red Hook directly. If you read posts from users cross-forum you will find that RH are not great with timely responses or responding at all. That aside, that is support outside of GOG that you must seek for a game bought on GOG.
It is fair to say it isn't supported here. It is fairer to say that in contrast to the support it gets on Steam, the support it gets here is practically non-existent.
Achievements and Mods have no bearing on any of that and for that matter the selection, sorting, accessible instructions and vocal community on Nexusmods (and others) isn't exactly what I could call 'hard to find and install'