Personally, I think that these aliens have been on Earth (more specifically, in Japan) for years and they used to be more active in the past. They'd be the origin for the creatures in the myths.
Originally Posted by Cali
Vol 23 Omake:
Nocturnal Parade of a hundred demons/aliens on Gantz.
The various types of aliens in the Osaka arc show great agressiveness. It seems they have adapted they appearances to those of youkai from old myths. This volume Extra will show the original characteristics of some youkai and the aliens who have assumed their shape.
Characteristics of this alien: It's as high as a bulding. It can spit an acid substance.
Characteristics of this alien: It emits noises and a [Ku Ku Ku Ku] sound while quickly moving with its snake-like torso. Its sharp claws are able to cut through the suit as if it were made of paper.
Characteristics of this alien: The first alien they ran into. It enjoys eating brains.
Characteristics of this alien: It screams [Uoooo] and it emit whispers while moving. It can attack sending off little spiders as high-speed projectiles.
Characteristics of this alien: It can expand its size. Attacks using its sharp tusks and quick movements.
Characteristics of this alien It can expand its size, the Gantz suit strenght has no comparison to the terrifying strength this alien has shown. Due to its incredible resistance, killing it wasn't an easy task.
Characteristics of this alien: Has taken on the form of an elderly man, a woman, a head and a giant monster, each time it's stricken down, it regenerates due to his incredible lifeforce. It's known to be a 100 pointer, but up to now its weak spot remains unknown.
(Translated from Shani Andras' post - about a month ago ^^')
So the youkai are aliens alright.
It seems there was some more info on those pages, but they're about the original myths rather than the Gantz version.
I know two things.
1. This cover page is awesome to the point of me wondering if Oku didn't have an intern draw it.
2. This fight will not end in the next few chapters. It will continue for no less than another 93 chapters at which point Nuri, having killed or maimed everyone but Katou will simply fall over dead before delivering the final deadly blow to our new hero. At this point Morgan freeman will appear and in broken Japanese explain how the microbes of our biosphere infiltrated and shut down the body of this extra terrestrial being.
Bring it on.
No morgan freeman, at this pace nuri will die of exhaustion.
lol at Caravaggio.
Then Jaoquin Pheonix will run in with a baseball bat to deliver some justice to Nuri mumbling "keep on batting".
Then M Night Shyamalan will reveal its all going on in Kato's head as he dies from 1000 arms, Jacob's Ladder style.