227 spoilers follow.
You'll have to bear with me; this is a little long...and I've got numerous points. Please read it all; it'll make more sense.
Now we know that Gantz has a large web following that investigates it. More importantly, we know where Gantz came from, or at least another place where it is: Germany. The name of Gantz is starting to make some sense.
Now let's look closer at the images of the giant black ball in Germany.
First, look at the file name: "Heaven's Door". This is clearly the name of the group in the picture...there are many of them. Heaven's Door...Heaven's Door...sounds like a cult, eh?
Think about the name. Heaven's Door. When would one be at Heaven's Door (we're assuming temporarily that Heaven exists...let's not start a theological argument; I'm talking from the perspective of the cult...I don't believe in Heaven myself)? When one...wait for it...dies. Doesn't that name sound more significant now?
But enough about the name. Let's think more about the image itself, of the people surrounding the giant black ball. Does it remind you of anything? Look at these:
See where I'm going with this?
Now let's investigate the Black Rock of Islam. It was a religious artifact even before Islam; Islam states that it fell to Earth thousands of years ago. It was originally pure white, but turned black after absorbing the sins of man over the years. Interesting, eh?
Now let's apply that to Gantz. Remember how each volume begins? That quote...the one about the light and the darkness. Now compare Gantz to the Black Rock. The pieces are starting to come together.
So what IS Gantz? Well, there are two clear possibilities based on this most recent chapter:
1) It's a replica of the giant Gantz of Germany, created by the Heaven's Door cult.
2) It fell to Earth or was spawned by the giant Gantz; no humans were involved in its creation.
Either way, it stands to reason that there are numerous Gantz's in various nations, Japan's Gantz being only one of them. Hell, who's to say there isn't a Gantz per large city? We can't know how many of them there are, but the big Gantz implies other Gantz's besides the one we know.
What is the purpose of Gantz? That's a tough one. The whole "taking on the sins" thing implies protection, eh? Gantz exists to protect the Earth. It's a security system. It literally takes people from Heaven's door and has them fight for mankind. There's even a Jesus element going on-Innocent people (well, not all of them) are sacrificed, brought back to life, and must go through a figurative "hell" to help humanity.
So what IS Gantz, as a story? It appears to be a giant metaphor for Christianity, Islam, and perhaps Judaism. This is nothing new in manga, mind you...numerous series have used Christianity and Christian themes in the past, such as Angel Sanctuary and Devil & Devil. So don't get all up in arms over this...I'm not saying Gantz is Christian propaganda; merely a metaphor.
So there it is for you...the truth behind Gantz, and what it stands for.