Interesting - I have (fortunately) not run into these issues (yet), but found a list (with specifics) at https://www.gog.com/forum/general/drm_on_gog_list_of_singlep...

Most of the listed games seem to be playable but with some content locked behind DRM, galaxy, or online connectivity.

Overall it does seem like they are trying to live up to their DRM-free claim/promise, but it looks like there are still a few annoying exceptions.