I'm half though the campaign and did not get a single achievement. Are achievements just completely broken in GOG or is there a other explanation? And btw: How do I view available achievements n GOG Galaxy 2?

Someone report that some achievments broken...
Also... Achievments mostly for Survival mode. So everything OK for you i guess. Statistic achievments - scores and infected killed - are counted only at the end of campaign. There is no else achievments for campaign...?