Maybe it's a difference in erm... strategies. Like, Osaka team is rampaging now, having fun etc. But on this particular mission they would suck cause of this, so really the tokyo team is sent there to help them.
The other point is that Gantz wants to show Tokyo team the way Gantz should be 'played'.
The 3rd: maybe there are not only these teams in Osaka, like there could be an other team they didn't met just yet, or another team which is in stealth mode (yes, the whole team of Nishis); so it could be the beginning of gathering of National Japanese team, before the worldwide action.
And there is always a possibility, that Gantz just had no shit to do, so he wanted to watch Gantzer rip each other asses, What would happen if not for Reika's breasts ^_^