I gotta wonder whether or not gantz is realing "saving" anyone's life in the first place. Remember that it's already been discussed in the manga that the people who are ressurected in the game are more like "copies" then the original.
This is especially true when you look at Kei, where gantz messed up and thought that she was dead when in truth her original copy was still amoungst the living
Overall though, I don't think Gantz is generous. Most of the time the people he brings into the room have no idea of the mission in the first place, and often re-die in new painful and terrifying ways. It's like reviving a person from a heart-attack, putting on a gorrila mask, and giving them another heart-attack.