He lost because Zaraki's physical vessel was beyond his ability to handle without imaging that he could before hand. He dominated the whole battle until he tried that. That's how and why he lost. When Zaraki has city sized meteor strength with one arm casually, I can imagine that's a problem. If he pulled that with Kaguya, it wouldn't apply. P.S. the vanish is not used as BFR. It's used to appear and disappear instantly and it causes the opponent in question to forget about their very existence. If Gremmy opted to use that, Kaguya would be a sitting duck. As he creates the ability, it applies to himself as well. If you want to argue that point all he'd have to do is have Gwanael Lee do it for him. P.S. In character, Kaguya is even worse than Gremmy as far as stupidity goes.
You are wrong, Gremmy admitted that his abilities were flawed and that he had definitely lost to Kenpachi because of how limited his Visionary was, Gremmy failed to imagine a vessel strong enough for Kenpachi and therefore lost out of the fact that his abilities were limited.
Kaguya wouldn't be a sitting duck because all she would have to do is escape via BFR'ing herself, how is Gremmy going to BFR someone that isn't there?
I wasn't referring to the Gweneal but his dimension pulling ability that sends an opponent into space.
Kaguya isn't limited, she's far more versatile and stronger than Gremmy in everything and has the key powers to one-shot him or kill him.
What stops Kaguya from impaling Gremmy with her bone spears?
You can't base a characters strength on his power set, even though his ability is haxxed, he have not shown feat even close to Kaguya level. These hax character often have limitations to keep them in line with other characters in the series, and Gremmy have shown them. Gremmy doesn't even have feats to prove that his power could affect top tier Bleach characters, so you can't assume it can affect Naruto top tiers who are leaps and bound above Bleach top tiers. Gremmy isn't even close to the strongest stern Ritter even, all of haschwald's royal guard are stronger than him.
You are wrong, Gremmy without a doubt was the strongest Sternritter, the guards nor Jugram are stronger, in fact, Jugram was panicking when Gremmy had envisioned the meteor.
Gremmy certainly lived to his hype as being the strongest Sternritter.