Gremmy specifically said if that were possibly not it was possible. But of course he will die somehow but I think I know how. If you are familiar with the Novel which only is partially translated, Zaraki became sort of hax proof like a lesser version of Dark Schneider's dispel bound. Kubo said that the novel was around what he had in mind for Zaraki or words to that effect. In other words Gremmy probably won't affect him. That may be why he messed with Yachiru and Gwanael and not Zaraki. He can't. In the novel nothing worked on Zaraki and the narration stated that the likes of Baraggan's time manipulation, vodoo, possession and numerous other hax couldn't work. He proved much of it too. He already incorporated one aspect, being that his reiatsu was too much to be felt. As noted by his men, they could not sense his reiatsu until after he began fighting. This is another point mentioned in the Novel which Kubo incorporated into the story as mentioned by Unohana. If he is too strong he will tend to lower his reiatsu to fight a weaker opponent (Gremmy) and if they are stronger, his power goes up to their level. In this case, he likely lowered his reiatsu to match Gremmy.