I was wondering if I should expect the achievements to be added to the GOG version. Does anyone know? Would I be more likely to get a response on the Steam forums?

You probably will have more luck there, since one of the devs is keeping track of that forum and replying here and there. I don't use Galaxy so I'm not sure how widely supported achievements in it are, unless the game itself is designed to keep track of them like Shovel Knight does.

3 years later... still nothing.
Welp, guess I could wait another 3 years.