User blog comment:B214/Volume 23 discussion blog/@comment-9595671-20170318111054

Ah, it seems that Gabriel doesn't actually appear in Dulio and Issei's match directly, Dulio used that bubble technique of his to give Issei a personal show of Gabriel for him alone. Though everyone did hear her voice from it.

That makes sense since right now Heaven's probably in a state where Gabriel wouldn't be able to participate even if she actually wanted to. I wonder what else Dulio can do with Zenith Tempest's bubble tricks?