User blog comment:Xilinoc/Ch. 574 DEATH IN VISION/@comment-387500-20140326185006

Seems to me that Gremmys ability only works if the opponent believes it. Such as how he told Yachiru about her bones being cookies and her then imagining it or how he told Isane that the captains were dead before we even knew. I think he can create an illusion which becomes reality once the opponent believes/imagines it themself.