How exactly do we unlock the costume chests in the GOG version? All the guides I've found on the subject say that they're unlocked by way of achievements, but the GOG version doesn't have achievements.

Sorry for the Necro, but I also came here looking for the answer. Turns out, the achievements they are talking about are in-game, not platform achievements like Steam. So everyone should be able to get some of the costume chest keys by playing the game. Cheers!