Steam has around 129 achievements. Why is there no achievements for the GOG game?

Probably because it needs someone to code them for the Galaxy client, just as it required someone to code it for the Valve client; the difference is that Steam is the market leader, and hence has people willing to make the effort because more / most people will benefit from it, whereas those who purchase games from Gog include a large proportion of their customers who do not even use the Galaxy client, and so it is not very highly esteemed (at least not esteemed by many people, as a ratio of customers who are able to use it).

Additionally, some of the existing achievements that have been coded for Galaxy do not work correctly; this is no doubt because there are issues interfacing Galaxy with the hooks that have been built into the games to transmit goal achievements to the (Steam) client.