Once again we have to do GOGs work.
What is GOG's work in this case?
At least GOG is probably not trying to track all GOG games, whether they receive any updates elsewhere. That would be quite an impossible task. Valve doesn't track either how games on their store are being updated elsewhere, and whether the Steam version receives all the same content and updates. And no one expects them to.
Now, if you people have informed GOG about some missing update, THEN I agree it kinda is on GOG's turf.
What can GOG do if the developer does not want to bring the patch/DLC or thinks the patch is not ready for GOG?
Bride them? Send a monthly email asking if the patch is ready. How many resources would it take GOG to focus on ensuring games get updated??
Urbek City Builder seems set on not bringing the DLC/update to GOG. If that's the case, we or GOG can only do a little.
What MarkoH01 said seems to be the only thing we can do.