Actually this game had listed the Galaxy achievements when it was released. I contacted the publisher as well as GOG and the publisher unfortunately told me that this was a mistake by GOG and that their achievements are not compatible with the way Galaxy does its achievements. Adding achievements on GOG was not planned by the devs (06/2017). Maybe they've changed their mind but I highly doubt it.

Here is exactly what they (aspyr) said:

"Achievements have never been supported in the GOG version of the game.
GOG has it's own entitlement system (Galaxy) that does not work with the game.
This feature has never been supported in the GOG version of the game.
If GOG is advertising achievements as a feature of this title, they need to change that."

"To follow up:
I do not see anything scheduled to indicate we will be patching the game any time soon."

"Sorry for the confusion.
From what I understand of the issue, we had said we would be open to working on achievements in the future, but they have never been implemented.
I will talk with our sales team to make sure GOG follows up in making sure that they make sure to avoid selling the game with the promise of Galaxy achievements."

Thanks for the reply, it's a pity that they have given up on adding them, perhaps I could contact the publisher and show them that it's something we want in many, maybe they could add them later as they are doing with many other games here on GOG. Where can I write them?

Here's the answer for you: Achievements are for morons.

You want your achievement? Write "You beat the game" on a post-it and stick it to your monitor. You'll get the exact same effect.