If you liked the Gantz manga, the Anime-Planet community thinks you'd like:
Both of these are about high school boys who are thrown into a life or death game not by their choice. They are both suspenseful, well drawn, and have sci-fi type weapons. While reading, I could not help but to think how similar these two are.
Middle school student Ganta Igarashi witnessed the slaughter of his entire class by the mysterious 'Red Man', and as the only survivor, he was labeled a mass murderer and sentenced to death. Now an inmate at the privately-owned prison and ‘amusement park' known as Deadman Wonderland, Ganta must try to survive in a place where inmates are the main attraction in brutal gladiator-style games. At the same time, he must attempt to uncover the truth behind the ‘Red Man', his mysterious childhood friend Shiro, and why he was the only survivor that bloody day.
As the blood splatters and the opponents get tougher, survival becomes increasingly harder. Gants and DW are centered around a group of people who desperately fight for their freedom. They must endure trials placed on them that, to a normal human, would be physically and emotionally impossible. Ganta and Kei started off as total wimps. They would of never been able to fathom the killing they would soon do. As time passed and the strain grew, our young protagonists matured into leaders. They become what everyone follows for survival of the never-ending hell. Massive gore, development in about everything and just a unique story, you can't go wrong in choosing one of these.
For centuries, vampires have lurked in the shadows of England's streets. The Hellsing Organization, currently led by Integra Hellsing, has spent centuries fighting these monsters and keeping their existence a secret from the world. Alucard is a vampire sworn to serve Integra and follow Hellsing under her rule; Seras is a fledgling vampire subservient to Alucard, learning about vampire ways while struggling with her own lost mortality. Together they keep England safe from blood-drinking monsters, until a new breed of pseudo-vampire, the Freak, begins attacking people for fun and not to feed. Where are these Freaks coming from and what will Hellsing sacrifice to stop them?
Recently the number of attempted suicides have increased dramatically, straining the health care system. To combat this problem, those who try to take their life are placed on a special island where they must live out their days, while the rest of the world believes them to be dead. Some respond by immediately committing suicide while others decide to try to live; but with no supplies doing so will be difficult. Now, this group of strangers must work together to survive on Suicide Island, or die trying.
What is the true survival? Is that hard to live in society? What is our purpouse in life? Just to wake up one day and find yourself far from the life you despised so much, fighting for surviving. This is the kind of situation the protagonist are fated to in these manga, where skill, nakama and the will to go forward is what matters the most.
When high school student Ageha Yoshina finds a red calling card in an isolated, ringing pay phone, he believes he's merely stumbled upon the game 'Psyren', a curious urban legend. He initially intends to trade information about Psyren to the world's foremost fortune teller for a famous reward, but after his childhood friend Sakurako – who happened to possess one of the cards – disappears, Ageha rushes into the game's dangerous world to find him. Now trapped in a post-apocalyptic landscape and pursued by Psyren's deranged inhabitants, Ageha must fight to survive and unravel this world's mystery before the game takes his life.
Both Gantz and Psyren begin with normal people being dragged into a deadly, inexplicably game - Gantz's in Gantz and Nemesis Q's in Psyren. The only way out of the game is to survive long enough and earn enough points to get out.
As the stories continue, both sets of characters gain more powerful abilities and eventually stumble on threats to the world as a whole.
Tons of action, monsters and mystery, if you you enjoyed them in one, you are sure to enjoy them in the other.