Ancient-Red-Dragon: That's not fully true. There is nothing stopping GOG from implementing a policy which insists that games which have Achievements on Steam shall not be sold on GOG
unless & until the devs first implement Galaxy Achievements for the GOG version as well.
Yet GOG does not do that.
Luckily so. I couldn't care less about achievements, and I want to buy games from GOG even if they don't have achievements. If you don't want to buy GOG games which lack achievements, then simply don't. Don't try to decline others from buying such games.
It is similar as demanding that GOG shouldn't sell any games that don't have French localized version (when the Steam version has). Why should all non-Frenchies, who couldn't care less about the missing language, be declined from buying the game? The same goes to a missing Linux and/or Mac version.
As I am sure someone will again come up with the "so should GOG sell games with DRM too?", I've said over and over again that it is their decision. if they feel people would buy games with DRM from GOG (instead of getting it from Steam with the same DRM) and it wouldn't hurt their profits, then go ahead. I personally don't see them doing that though as DRM-free is one of the major things that differentiates them from e.g. Steam.