Yep, I agree that the idea of a fat retarded girl being the main force behind the creation of gantz balls is without a doubt stupid but this is only how the balls are created. I don't give a damn on how they were made I just want to know why. The guy mentioned it being about a bet but I hope he's joking.
The writer f*cked this up but hopeful there is a better reason behind it or the whole series fails.