zavlin: how would you manage achievements without a client?
Ask every dev to make gog specific achievements and put them only in the gog version of the game, and then say "you figure out where it gets displayed, dev" ?
Its possible, but it doesnt make any sense if the achievements arent unified in some way, and if you start to unify them it will start to look like a client.
And if you dont use online drm, then theres no way to even begin to ensure the achievements are valid and not cheated in.
Code to handle the GOG achievement system would be required regardless of whether the game itself talks to GOG's servers, or merely talks to an external application that talks to GOG's servers, the former would just require more code (either idea would likely be provided by a GOG-made DLL that the game simply calls functions from - but the game would need to load the DLL and call those functions similar to what Steam games do).
Steam-only games like Half-Life 2 and Portal can be played completely offline and completely without the Steam client installed, but only register achievements if you're logged in to the Steam client.