For those still waiting for a proper Edited Manga, you can read it online here at
http://mangahut.com/manga/Gantz/285
Well seeing the Edited Manga they have, not entirely sure if they translated it correctly, my guess was partially correct, on the part that humans from Germany are involved. But...
Well presumingly, if im the Nazis, i dont realli care who i enslave so long as its not my own people. As a fact, if im a Nazi, i'll prefer sending Gantz to the countries i dont realli like in the first place so as to screw the people there. Everyone controlled by Gantz are slaves anyway.
On...
My 2cents,
Germany is chosen for a reason, and the most infamous stuff there will be the Nazis and Hitler. So my theory is this, Gantz was created by the Nazis using some sort of alien technology they acquired during the Hitler period. Using the technology they want to revive the Nazi period...