Hi, sorry to hear you had that issue. We looked into it and even though it appears to have worked before, it stopped working for us as well. I republished the achievements on GOG, and now it does work for me. Please check again and let us know. Thanks!
Weird. It works on Steam and it worked when I tested on GOG. We don't have any different code for GOG because GOG uses a wrapper for the Steam achievement code. So this might be some weird GOG issue. Sorry.
Wait, did you fully close and reopen the GOG launcher? I did that and the launcher updated when it loaded and then I got the achievement. Maybe try that if you haven't already.