User blog comment:Xilinoc/Ch. 574 DEATH IN VISION/@comment-24113610-20140326174221/@comment-11051098-20140326202408

Gremmy's power probably works by becoming reality when someone believes his words. Isane was shocked to hear Rose and Kensei were already dead, and when she rushed to check on them, it became reality.

Whether Gremmy's imaginations/visions becomes the permanent reality or only as long as he stays focused on the target, remains to see. Yachiru didn't appear to have cookie bones when Kenpachi carried her, but it's too soon to tell as we only saw 1 good panel.