Okay, uh... long ass post, I apologize in advance, but my two cents:
Took a quick read at Gremmy and Kenpachi's fight, and while I don't disagree that from a writing standpoint it could have been handled better, the way Gremmy was defeated actually does seem to fall more into his own personal mental shortcomings and flaws than anything, more especifically overconfidence.
I mean, when Kenpachi challenged him he did seem eager to amuse himself, he did choose to try to overwhelm Kenpachi with sheer power as if to show him up, was unable to imagine a level of destruction that could overwhelm Ken thinking whatever he came up with always would be enough, when pressured his imagination and confidence to imagine positive outcomes started to falter.
It doesn't come as him being a fool, rather as being too sure of himself and his admitedly overpowered ability. And that's the issue, Refia due to the tier difference has much more raw power than Kenpachi had, but more importantly she can actually resist many of the immediate effects Gremmy has though of with the Visionary; Instant Death, being erased from existance, and even turned into cookies (it's not properly detailed in her profile, but her resistance to Status Effects includes Transmutation). If Gremmy tries to simply dispatch Refia he isn't unlikely to come across an effect that doesn't work on her, if he tries to simply overwhelm her she can surpass many of the scenarios he thinks of and all of this will slowly crack Gremmy's confidence.
Gremmy also chose to engage Kenpachi because he thought Kenpachi was strong, so a fight isn't that unlikely either. And let's not forget Refia's abilities which, aside from Confusion, may not leave a permanent effect on Gremmy, can still help her fight back offensives thought by him and her sheer destructive power may intimidate him similarly to how Kenpachi did.