Gremmy thought of Kenpachi as the most unimaginable monster he "could think of", so Naturally all riled up as he was, Gremmy thought that it would be easy by thinking "I'll make myself stronger than such a unimaginable monster" ... but what's stronger than a monster that's already unimaginable?
In other words, the kid started thinking of Kenpachi as even stronger than he really is, beyond Gremmy's imagination even. He imagined Kenpachi being beyond his imagination.
Gremmy lost because he doesn't have control over his thoughts and imagination. Which he can't really be blamed for, nobody really can without decades of meditation and such, and even then it's difficult.
That's the ultimate drawback to such a power - your self-control. With this power it's easy to end up in a spiral upwards, or a spiral downwards. It's basically a power that can, for better or worse to the user, self-perpetually escalate out of control.
Basically his body tore itself apart trying to give it self a power up that could never be attained. Because in his head Kempachi is the strongest thing he could imagine.
His Imagination literally ran wild to the point he defeted humself.