That guy whose head explodes was a warning to not switch in and out of normal human frequency, and making Tae the target is to remove witness (though Gantz could just be trying to make Kurono, and the others time out without eliminating Tae and have their points reset).
One thing I don't understand is that the aliens seem to be only hostile in the area Gantz sent the suits to. Onions, Shorty, and Buddha were all pretty peaceful, and only started attacking after the black suits appeared and messed with them. Gantz might just be a part of an extremely advanced alien civilizations who is trying to take out the other alien races without involving their own people before invading earth.