User blog comment:Fallenbladr/The Elite Guards Plot & Stuff/@comment-24165551-20141106045423

Yhwach probably found Gremmy to be too difficult to control to make him one of his elites. The guy nearly destroyed the Seireitei with a comet. Yhwach can't afford to have that kind of loose canon nonsense going on while he's fighting the Royal Guard. Gremmy was a blunt instrument whose purpose was to neutralize Zaraki.

Honestly, Gremmy was tremendously OPed, and if he had been sane and intelligent, the Ritters would have won instantly. I mean, why wouldn't he just imagine every single shinigami dead? War over, Yhwach wins. But no, the godlike powers always go to some psycho kid who ultimately loses from a lack of common sense and (ironically) imagination.