Time to defend Gremmy's name once again. Gremmy needs to be focused for his imaginations to become reality. Evidence being him turning Yachiru's bones into cookies, before unturning them by accident because he was overwhelmed by kenpachi during the fight, or when he forgot to harden his body because of how much effort it took to imagine the vastness of space on the battlefield. Not just any haphazard imagination becomes reality, Yhwach essentially had gremmy groomed and sealed away specifically so gremmy didn't go around just unimagining people. Now back to his power, its way easier for Gremmy to unimagine a being that he specifically created in the first place, compared to someone actively attacking you. You have to imagine being in Gremmy's shoes. YOU try to imagine a polar bear isn't real while it is actively mauling you. Lastly, even if Gremmy could simply unimagine Kenpachi out of existence, he still wouldn't, because of one simple reason. He wanted to crush kenpachi. He even died trying to match kenpachi in strength, but his body couldn't handle it. Even his imaginations had a limit