When you're bored, you think about these things
In attempting to justify why Gantz does what he does there are a few things that are hard to make sense out of. For example, if the point is to kill aliens and protect earth why not just send a minute man out into the streets holding a decapitated alien, yelling his lungs out about it?
If its all gotta be "super secret", then why don't they just clone all the most successful gantzers from the past as many times as seen fit (perhaps many duplicates at once) and keep them somewhere, oh, I don't know, "super secret"? If its all about killing aliens then why even use clones of people that go back and forth between the rampant slaughter and some semblance of a daily life or use clones at all, and why give them the option of leaving or resurrecting others?
Well the answer came to me and it encapsulates itself quite nicely in one sentence (all caps):
ITS ALL ABOUT THE PRIME DIRECTIVE BITCHES!!!
So some alien environmental-conservation-esque organization looks at earth and goes... "0H N03Z!!! 73h u8er r16ht w1n6 fu|\|d4m3n7al15t 4li3n5 4re 60n|\|a bl0\/\/ up pl4n3t p4nd4!!1!une!!"
If leet anoys you: "Oh no! The uber right wing fundamentalist aliens are gonna blow up planet panda!"
The organization's leaders assemble at a hotel in their nation's capital "Quake II" and prepare to lobby the current minority government with their concerns, primarily for stronger environmental safeguards, and against the proliferation of nuclear legs (that's just how advanced they are).
Given the fact that the environmental buffs are the leading party's primary voter base, a coalition is formed between it and the nihilist party; the prime minister promises to send them a representative's foot.
Unfortunately for the conservationists, in lieu of the almighty and recurrent "PRIME DIRECTIVE", the government cannot directly impose itself on planet panda for the protection of its endigenous wildlife. Therefore, they hire a panel of researchers to assess the best way to go about blowing up the illegal alien emigrants and building contractors (as is in accordance with their law) that plague planet panda.
Millions of wasted tax dollars later, the panel proposes that instead of directly interfering they gently persuade the humans into doing the task willingly (so as not to be tried in the act of coercing a lesser civilization in defiance of the PRIME DIRECTIVE) with severe consequences imposed on participants for potentially exposing the rest of the human race to the developements of further advanced civilizations and breaking the PRIME DIRECTIVE themselves, exemplia gratia (e.g.) blowing their fucking heads off (a common legal practice).
In order to ensure that the task is taken willingly by the participants the panel devises a prize system for those who reach a certain head count allowing them to opt out at that time or to take a further interest in the protection of their panda haven.
Though simpler solutions were offered, it was determined by the research panel that making numerous duplicate clones or selecting ideal human participants by any method other than random lot would be considered overtly interferential.
After a month of consideration and post-review the minister of defence issued the order to dispatch a preparatory unit carrying arms that were either earmarked for museum exhibits or to be decommisioned at an arms disposal facility. The unit randomly selected human candidates to operate the mission control centers (and shaved their body hair) so that the operation could be considered a human maintained endeavor (without any chafing).
Having contributed their collective influence the environmental lobbyists felt personally fulfilled and returned to their lives of rampant consumerism.
The representative died of infection and his family persued charges against the hospital's chief director.