I'm just now thinking about this but what if Kei has already beaten Gantz. The reason i say this is what if when he was really young he somehow beat Gantz. That would explain why he did all that crazy shit when he was young. It also explains why he stopped doing stuff like that: his memory was erased. This even makes more sense concidering that for some reason he "knew" to take the suit and it explains how he adapted so fast.
What do you all think about this?