According to my dictionary the definition of gangrene is: "death of body tissue usually from a loss of circulation." If in fact the conditon in your son's deer was gangrene, or just and infection, bruise, etc. then you did what we have done in the dozens of deer that we processed over the years that had similar injuries. We discard the dead body tissue/injured area and utilize the unaffected area.
I see no reason to discard a whole deer to a localized injury.
Dave