Still didn't work. I went into my save where I had a magic missle and it still didn't work. I tried the multi fireball one too and that also didn't work.
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.