Ancient-Red-Dragon: I'm not sure why the OP is high rated. I downvoted it, because it is cherrypicking which types of failing to support a game is unacceptable and which types are acceptable. For example, I don't see any commentary in the OP complaining about the legions of devs who release games on GOG with no Galaxy Achievements even though those same games do have Steam Achievements. So those devs aren't giving GOG customers an equal game with equal features either.
As far as lack of GOG-specific achievements are concerned, that's the result of devs choosing to tie them in with Steam in the first place, ie, had they made achievements "internal" to the game (like Dragon Age Origins), they'd be present and naturally supported on every platform (inc even physical retail disc or client-less offline installers). By making them "external" (ie, locked into store-front specific API's), they become more 'social' to that platform, but it also multiplies the effort in supporting multiple store-fronts. This has always been the paradox of supporting achievements on more than one store-front - because gamers want to compare them 'socially' online, they don't want the "easy way" of doing it (coding them in-game), they want the "hard way" of locking them uniquely to each store front then having to go back and recode them for each additional store-front client they sell on.
https://docs.google.com/spreadsheets/d/1pDO6WTHLHyrrtidQ1MAxW6u8j3BxUaGcFaJsVyWj2QY/edit#gid=0 ^ Merely looking at that list, removing 800 games from GOG as you seem to suggest would shrink GOG's catalogue by 25-30% overnight. Many games have technical limitations, eg, Slipstream, The Count Lucanor, etc, can't have them as GOG SDK doesn't support Java. For Artifex Mundi, it would mean rebuilding some games in a new engine.
Lack of achievement parity may be 'equally unfair' but it also involves a lot more duplicative work than patching a bug once for all platforms simultaneously. Likewise I'll be blunt and state the reason Steam invented Steam-only, Steam-locked stuff like Steam Achievements, Steam Workshop, etc, is not out of charity. It's to habitualize devs into using it by default then getting them to feel that it's 'too much effort' to support additional non-Steam stores. And in that respect it's sadly been "mission accomplished".