If Gremmy Thoumeaux can imagine anything and make it really happen, and if he's really that mad, why didn't he just imagine Kenpachi dying without any talk and fights?
Gremmy lost because:
He imagined that he was as strong as Kenpachi but he failed to imagine that his body was strong enough to contain Kenpachi's power and thus the power tore Gremmy's body apart. Furthermore, Kenpachi also caused Gremmy to doubt himself and imagine his own defeat and death. So Kenny won because his imagination was greater than Gremmy's! Irony!
And this is speculation on my part but Gremmy doesn't seem to have the power to will someone dead. Certainly not someone as powerful as Kenpachi.
Gremmy was always creating physical objects to destroy Kenny (meteorite, guns, missiles etc.). The exception was Yachiro, but Gremmy touched her to turn her bones into cookies; I'll have to review the battle to see if Gremmy ever touched Kenny.
And the out of universe answer is that it would be a pretty lame fight if Gremmy could just imagine his enemies to death. So there has to be some challenge, drama and limitations in play.