Gantz/WMG

The story is based on the author's traumatic game of X-COM.
Anyone who has played a nighttime terror mission can attest to the cycle of overconfidence -> sudden deaths -> confusion -> people running around in stark terror and dying horribly -> Total Party Kill. The author is just fictionalising events slightly. Hell, even the guns and battle armour look similar.

Gantz is a reality show for a bored omnipotent being
Think about it. Futuristic looking weapons and suits could not possibly be made with current human knowledge. Gigantic pods that give out points for the number of points being scored. An approaching apocalypse. It would definitely make for one hell of a plot twist.
 * Pretty close! Turns out
 * Oh yeah, but that totally didn't seem dubious after Hmmm.
 * only after causing mass destruction out of thin air
 * judging by the attitude of said being, the entire alien invasion and gantz can simply be attributed to the fact that this "omnipotent" being has all but said that he is naught but the equivalent of a holographic projection from a higher dimension, and he is clearly just causing humanity misery For the Lulz. What else would a bored, limitless, being do with what is effectively to it a sentient version of Sim-City

Kurono survived thelast anime mission.
Note that his pic didn't grey out like the others. It's possible that surviving gave him 100 points and got him released.

The aliens have something to do with religion.
Think about it. The Buddhist Temple Mission had alot of figures from the Buddhist religon. The Oni are important to the Shinto beliefs. The Osaka mission was almost entirely composed of monsters from old Japanese myths and beliefs. In the Rome mission, they were attacked by statues of angels and other religous figures. Coincidence?

Concerning earth technology and aliens appearance.
The aliens invaders have threators amongst then,it's the only way for humanity to have access to that kind of technology,so sooner or later there will be an inside attack.OR they could have made the most terrific act of manipulation by giving humanity some of it's technology so the ycould take on the aliens living on Earth who could have been to only truly obstacle in their way,this way it also explains the behavior of the first aliens;The onion alien only attacked because the gantzers killed his son,the Robot Rock Lee's only attacked after Nishi killed one of its siblings/spawn,and the buddah aliens only attacked after the initial attempt at their lives after that rumors started circulating around the tokyo area about the alien hunters,so when Kurono appeared near the chibi alien it was already prepared to attack him, so this way the Invaders could herd out some of it's competition while thinking the humans would go down without resisting when they appeared what they did no expect was humanity crazy survivalist nature.