Sadly, the overlay is not supported for this game. I asked support about it when the game was released, and it doesn't look like it's going to be fixed because the Galaxy overlay just doesn't work with some engines or games. I wish it was as good as the Steam overlay which works in almost every game (even non-Steam added games). I hope GOG improves it, but I don't think it's one of their priorities which is kind of disappointing for achievement hunters.