My library gets an update notification all the time for this title. It`s been going on for months. What`s going on?

They do periodically release actual patches (they have been more common since the recent Final Cut update -- presumably, they're still working a lot of kinks out of that); not all of these are necessarily for all OSes, though -- i.e., some might only affect the macOS or Linux version, thus the Windows installers won't change, but everyone still gets the notification flag. Beyond that, some of them might just be GOG or the devs tweaking the Galaxy-related files, with no change to in-game content or functionality at all. This flags as a new update, too.
And, of course, some of them might actually be the "ghost updates" that have sometimes happened for certain games, where literally nothing has changed, but owners keep getting the "updated" flag anyway. :P

I can say that most of the recent update flags I've gotten for this game seem to have been related to an actual new version, even if only a minor bug-fix patch. Then again, I don't use Galaxy, and I don't usually check my library more often than maybe once a week, so I might well be missing out on a lot of renewed flags accompanied by no update.

Update notices are still popping up for the game even though nothing has been updated since 13. December.

It has, though. There's been at least 2 patches issued since then; it's just that the changelog hasn't been updated properly since the 13th.
Changelogs not being updated reliably has been a longstanding gripe around here for years. :/