 If you liked Gantz |  ...then you might like Tokyo Ghoul |
Similar dark and gory vibe to Tokyo Ghoul, Gantz removes the ghouls, adds aliens, more guns, and maybe a few less dead people. Definitely a good read if you're looking for a lighter, but still dark manga.
report Recommended by TheAlch3mist
Man-eating monsters and a ton of action. Gantz differs being a lot more violent with sci-fi elements. While Tokyo Ghoul differs with having a struggle of identity as the protagonist is transformed into one of these monsters. So, if your looking for a dark manga with some kick-ass action I suggest you read one of these.