Sounds like you are looking for the term “Historical Fiction”?? This is basically where the writer inserts made up elements into historical events, or simply just dramatises historical events a bit. A popular literary example of this is Wolf of the Plains by Conn Iggulden, or maybe at a stretch Norse Mythology by Neil Gaiman. I just got finished reading The Day of The Jackal by Frederick Forsyth which I would also loosely call historical fiction.
But then Gantz is pretty much just “Science Fiction” with dark elements. A lot of top class Sci-Fi is less about space battles and more about exploring extrapolations of where technological and/or societal trends/experiments/questions may lead. The Forever War by Joe Haldeman, I Am Legend by Richard Matheson, Neuromancer by William Gibson, Dune by Frank Herbert, etc.
There is a whole lot out there to read, and if you have a kindle or tablet then a lot of it you can find for free or dirt cheap on the Amazon Kindle Books Store by just sorting the list by “price: low to high”.