I've been looking into this, and what you said is true - this used to be near impossible. However :-) There's been some (very) recent developments - and good ones.
Would you mind taking a look at this post?
https://www.gog.com/forum/three_kingdoms_story_conussia/gog_galaxy_achievements/...
According to the included instructions, this is now very easy. You should be able to export/import the achievement list from Steam (this was possible before already), and with the included files, you should be able to make it work for Gog with no code changes.
Certainly worth a look IMHO.
BTW, I found your presentation on GIC on YouTube, I will be watching it :-)