In order to make a new body, Gantz needs the data from this guy. The data Gantz already got from this guy was "screwed"; that is, as he had already been suffering cancer for a long time his body had become weak because of the illness. So even when creating a new body, Gantz had to replace pieces of this "corrupt" data with fresh new data.
Ex: maybe this guy got a cancer because of smoking, so his lungs would have been pretty damaged. When the guy died the damage to the lungs had been done some months or years before and had evolved from that. So in order to create a new, healthy body Gantz must drop the data from the ill lungs and in its place use new data, that is: Gantz should have created on its own the data for this guys lungs...this isn´t as easy as it sounds, as the lungs can change a lot from one person to another so Gantz has to imagine everything. We have never seen Gantz do something like this again. This is not the same as creating a body for Katou or Kurono, because in this case, Gantz already has the data of how the body is in it´s healthy state. Also, Kurono got his old wounds cured like someone mentioned before. If Gantz was able to create this data on its own, then nothing would stop him from creating entire new humans...he could create an army of 1 million Gantzers!
Isn´t it more reasonable to think that Gantz just cured him?