>what would your first reaction be?

I would sit tight and gather information while looking for the exit. :)
For sure I wouldn't want to go into a battle unless I really, really have to. It takes years for the Army to train a soldier. Draftees are the worst kind of soldiers, they are the first to go down in a war.

>was their any other kinds of mass destruction in the manga?

Yeah, plenty. Almost every time.
IIRC the media/officials blame it on unknown terrorists' attacks.

>if Gantz provided more information, they proberly would of listened

But he does give info.
AFAIK Gantz does his little speech every time (You are now dead, you belong to me, you play by my rules, go and fight the following aliens, here is the known data about them, etc.)