Hi. It doesn´t work. The achievements only pop-up when I get them online on Gog Galaxy. As long as I am offline, they don´t count.
I rescued Gareth playing offline, for example, but the achievement didn´t appear as soon as I logged into Gog Galaxy. So I opened the game one more time (online, in this case) and loaded a previous save, did the rescue again, then it popped-up (only online, not offline).
Works fine for me as I mentioned. I have ~127+ hours of play on the in-game timer, but two hours 30 minutes on the GOG onlline Galaxy timer. My last achievement is the Arx landing, which I just did--seems fine to me.
What you might want to do is to run the game through galaxy and play for thirty minutes or so to update your achievements, and then go offline. I have seen occasions in which the achievements take a little time to update--maybe the GOG server is slow, hammered, being maintained, etc. As you can see, I am up to date on my achievements despite being offline 99%+ of the time.