Manga News: Check out this week's new manga (10/13/14 - 10/19/14).
Forum News: The nomination phase of the Community Awards 2014 is live! Visit new sections for Nisekoi and Kingdom!
According to the insufferable marketing copy that I'm forced to read on the title page of every chapter:
Gantz Mobile Mission is the "cellular game version" of Gantz/MINUS.
Gantz/BURST is a new cellular game where "The protagonist is you!" So I'm guessing it's just playing through the story as if you're a person trapped in the room.