I am pretty sure I didn't miss any secrets during my playthrough (played through it once before on Steam (using Simrock's guide) and got the achievement there). However it didn't unlock for me on GOG Galaxy. Has anybody successfully unlocked this already?

Same here.
I used the video guide from Cyber_Ninja, but the achievment is still locked.

Now (4 months later) I'm doing a complete run again, currently on level 7 "the lost land"

Edit:
Completed the game and it's still locked...

Edit 2:
Completed the game again in one sitting, without leaving the game: It doesn't work...

This happened to me as well. It has to be bugged.

GOG Galaxy currently flagged the first two achievements as completed for me while I had played it. I know for sure I got all the other secrets and did the things in the other achievements, but they are not unlocked for me.

Still didn't get it either. It's difficult to "hunt" for achievements when you don't even know what they ask for...

I contacted GOG about this issue and got a reply that they have forwarded the information to Galaxy QA, which they said will investigate.

Let's hope this gets fixed soon. I hate broken/bugged achievements.

Yep, it works. Finally.