To be honest, the backgrounds are not that good because Oku draws so well. I think he uses photos and transforms them with a programm into background art (Miura draws everything which takes much more time). Never was too clear on that though.
Whatever works is a good thing. And for Gantz it works pretty well.
And Oku's doing great your're right. Kurono is back, Reika is staring to loose it, Inaba finally died and Gantz is back to a more stable release speed. Hope he keeps it up.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"