If a game is started through gog galaxy, and i get an achievement, then it doesn't show that in gog galaxy (and doesn't update the achievement %), nor does it show up on the activity feed on steam, although steam does say it was "unlocked".

Achievements are a bit bugged right now, for me as well. None of my games have updated or posted beyond 0% besides Xbox, PSN, and some Origin games. Just report it in-client using the Report Issue button.