LootSeeker: I don't understand how this game is too niche for GOG. Wizardry, Wizards and Warriors and similar games are all here. A fan base for this type of game absolutely exists here. The "it's too niche reason" is a pile of bum poo. More than a few GOGies have been waiting for its release here (650 votes the last time I peeked at the wishlist), and while its rejection is annoying enough, the reason for its rejection simply......grinds my gears.
It's rather simple, actually. A curator, who replied (and apparently judged) about Grimoir rejection, was Dr.Cat - a developer of Furcadia game. His game is not on GOG, probably because it's too niche (you know a pixelated isometric MMO about furries is hardly mainstream), so I thnk he just decided that any game that he consider not as good as his, shouldn't be on GOG too.
Essentially, it all comes to what was stated in some comment earlier - as long as there are no officially stated clear defined guidelines for what games are accepted and what are rejected, the curation on GOG is essentially based on curators' taste.