Completely agree with tylerdurden and dickwinters. This depicts hell on earth without any concession. It is disturbing, sobering and very moving.
Like Grave of the Fireflies, it is a story of survival that knows no happy ending; so many of the efforts to keep each other alive were in vain as another main character died. I personally felt that Grave was a bit more subtle in showing the horrors but ending on a very pessimistic note, while Gen is very uncompromising and gruesome, but retaining a sparkle of hope.
Edit: added spoiler tags.