This manga for most people seems to be a love or hate relationship, It contains lots of extreme content, including gore, horror, psychological , and some sex. If you cannot stand any of those genres don't bother reading it, if you enjoy or don't mind them, then you may want to read this manga yet.

Gantz is a story about a 14 year old teenager named Kei Kurono, who among nearly 10 others dies and in the moment of their death found themselves transported to a locked room with a big black ball and forced to participate in a "game", In which they must hunt down and murder aliens. This story is shrouded with mystery, who is Gantz? why do they need to kill the aliens? Where did humans get this technology? Does god even exist? Gantz progressively gets more and more complex and continues to question reality and human existence.

Here is my personal analysis of Gantz:

Story 10/10: (I can't be that descriptive while avoiding spoilers)I found in the beginning I was interested right off the bat and while this was not a plot driven story, the idea was done well. In the beginning the only goal was survival, in the middle it was adapting, and learning to do more then just survive, to win. And near the end (FAIRLY OBVIOUS SPOILER) the goal is the survival of humanity and the main character going through hell and back to rescue the women he loves.

Characters 9/10: The main character Kei Kurono starts off as a pitiful loser, not the "ahh I feel bad for him." kind of thing, but more like "This guys fucking gross, what a dumbass pervert, you won't amount to anything." But while in the beginning you hate him (as you are meant to), he has the mind of a realistic normal highschooler, not super hero, nor secret potential, he's a regular guy. But he quickly develops (realistically though imo) into a survivor mentality, he goes from being pervy, desperate and weak to getting the need to be stronger, and the will to survive, he enjoys power but doesn't go crazy with it. Eventually he becomes a character who you respect, he becomes a cool guy. he doesn't suddenly develop extreme morals, or the need to save life, he just cares about others while he cares about himself. The other characters are not that major, and honestly most of the die and stay dead (permanently or for a while). First off Kato Masaru is a stereotypical good guy hero that was childhood friends with Kei and believes in Kei unrealistically who is used to contrast Kei being a loser. Kishimoto Kei is used to make Kei Kuruno seem like more of a loser and make him jealous of his friend Kato, who she likes, although obviously she turns out to be a fanservice (Big boobs) side character. Nishi Jouichirou is an obvious selfish monster who shows the true harsh reality of Gantz. Reika is actually important and is a stalker who loves the main character unconditionally and tries to get him to love her, in any way possible. Other characters not mentioned were either unimportant, died, or did not have a strong relation with the main character.

Art 9/10: The art was very dark and gory, it portrayed the exact atmosphere intended, the only problem I had was the shading was too intense at times, (ex: black suits, night time, black weapons, dark enemies), Sometimes it was a bit overdone, but overall the art was very suitable and good.

Action 10/10: The action was top notch, amazing fights, realistic pain and struggling, no extremely overpowered fighting moves or abilities or weapons, while the weapons may seem extremely strong, the enemies are also extremely strong, evening it out. The fighting allowed for any of the main characters to have a good chance to die, allowing Gantz to be very intense.

Romance 10/10: The "romance" in the beginning was of a sad guy trying to get laid and lose his virginity, Kei Kuruno is extremely desperate and has a pathetic non-existent love life, being so pervy he would do scumbag things (taking advantage to feel up a homeless girl), but then again he was never loved, ever. His parents disliked him, his brother ignored him, he was a loser to them. But eventually he finds love. Not a babe or a goddess, or a genius, or a slave girl, but a realistic homely girl that for once in his life gives him love. That means more then anything to him and from then on his love for her makes romance a major and beautiful part of this manga.

Enjoyment/Overall 10/10: I found this manga, in total, to be beautiful .The extreme realism in a sci-fi manga, the main characters development, the amazing romance, and finally the inspirational messages this story has in the face of humanities weakness.
If you actually finished this, thanks for reading, I just wanted to share my recommendation, because In my opinion this manga, was very amazing and had a lot you could learn from. (Note anime while good, is nowhere near as good as the manga imo)