Manga's still probably the only way for you to come out enjoying it even slightly, considering the manga's abrupt ending leaves a lot to be answered and makes many of it's events seem pointless. Not to mention the art's far better in the manga, since the anime suffers heavily from the typical poor animation and bland character designs you tend to see in most early to mid 00's anime that were experimenting with digital art, plus the CG is both dated and immersion breaking.
From what I've seen, Gantz is far from a deep or well written series, though the manga's good art and absurdity of many of it's events make it a decent source of entertainment. The new manga by it's mangaka looks like it could have a far better story, however.