I completed the game on Nightmare difficulty, getting every achievement before that. But my achievement list now sits at 95% because "Dragon Warrior's Quest 'Nixed (Complete the game on any difficulty.)" didn't unlock. I even got the "Taking It All Ta-Go (Obtain all other achievements.)".

Can the devs unlock the achievement for me, or do I have to wait for a retroactive fix, or do I have to complete the game in an easier difficulty (lol, I won't do that from scratch)?

All of the achievements worked fine for me. I started my initial play through on Nightmare and worked up to 100% completion with no problems on both Steam and GOG. Not to say this isn't an issue with the game, but I'd bet this is more likely to be a GOG Galaxy issue. You may want to contact their support. Or just reload your save and beat the boss again.

Nevermind. After rebooting my PC (real time hours after finishing the game) now it appears as unlocked. I don't know if it was a synchronization issue or the devs silently unlocked it for me (or for everyone with this bug).