First of all, I am a huge Gantz fan, so when I heard there would be a spin off, well let's say I was excited and hopeful. Boy was I disappointed and in a huge way.
The spin off is pretty much an insult to the original Gantz manga. The story is poor, boring and goes nowhere.The characters are dull, and honestly it's hard to know who is who, or care.
The fact it only managed 18 chapters shows how popular it was, i'm suprised it got that many.
Overall the spin off is an even bigger insult to the original Gantz manga, than the anime was.