^^ On the good and bad, there's the introduction to the volumes, which you mentioned.
The light and darkness dicotomy to me sounds like something that could relate to good and evil.
The thing is, I could take and actually find cool if Gantz were a "good guy", with twisted ways, and I could find nice if he's a god with twisted intents, but if he's a god and brought on about the catastrophe due to something like human evolution leading to a wicked society, then nop.
Also... no, I'm going wrong about all this.
What I really mean to say is: I'd like the aliens to be aliens, therefore, each of them having a different culture and beliefs, knowledge and goals. As for Gantz and "the word", I can apreciate the god thing, if it has any sort of different from usual aspect on it. (well, it already has if we think that no god has even been show to be a man inside a black ball)
^ About the milkshake man and his machine, Oku shot us off; imagine the potential for meched Oka fighting the Nurari...
Oh, all the things we'll never see...