To capture one of the aliens and inflict some damage.
A group known as the Ziabusto group hacks all the Gantz balls in Japan, and transfers all the players to a centralized undisclosed location. The group themselves are located on a circle plinth surrounded by multiple Gantz balls. You can see one of their members using a small laptop that appears to be directly linked to a Gantz ball. The leader of the group then announces to the assembled Gantz players that they are tired of suffering the loss of military assets and plan to attack the aliens on their home ground. This is met with disapproval from various Gantz players one asking what if we refuse to fight. To this the leader of the Ziabustso group declares that they can and will detonate the bombs in the players heads if they refuse to take part.