GoG gets the same achievement list as Steam - So, it's really up to GoG to implement the achievements, not Larian.
Proof: If you check games between Steam and GoG you will find the same list of achievements with the same requirements. The way GoG implements them isn't as good as Steam but that's neither here nor there.
Achievements list are often (almost always, in fact) shared among ALL platforms that support achievements, because the list is decided by the developers.
That doesn't mean that it's GoG's job to implement them after they are decided. It's usually still up to the individual developers, but the platform holders usually have to offer the tools (i.e a specific API) and support to make the process as frictionless as possible.