Gantz will always be one of my favorite manga ever created.
I will try to touch a different subjects for this review - hope you will enjoy it.
Minor spoliers in the first half of the review.

This manga explores human nature in an extreme condition.
The Story starts as a survival game with the awful horny main character Kei Kuruno and his friend Kato, that instead of dying like 2 complete idiots to a train - they "teleported" to a room with a black ball and alot of confused people, after a few minutes of a vague explations by the black ball they call "Gantz" they sent to a random location with aliens, that they have to kill.
If they kill enough, they can earn points, with 100 points they can be set free from this murders game, revive someone, or gain a new weapon.
After each successful mission, they will allowed to go home, and will be teleported again in about a week for a new mission until death/100 points

The premise is great, interesting concept with great sci-fi designs like the Gantz's suits, the hard suit the huge mech and the cool guns - you would not be disappointed.

The gore is extreme in this manga, and there are nudity everywhere - so yeah, NFSW.

I won't spoil the story, but its a huge roller-coaster of emotions, you are in for a ride!

Now for what I didn't like about Gantz, because any story have flaws.

First thing I would like to point out, this manga is racist, and homophobic.
Black people in this manga are usually bad guys, Izumi painted his face black and goes on killing people, the gay black guy in the start of the manga that want to beat up and... fuck Kato, I'm sure I can find more examples but yeah its a racist manga, hope you can overlook it.

Its Homophobic, if someone is gay, everyone immediately gets away from him, even one time a character scream at a gay person to get near him.

2nd thing - and its a real shame:
Women are objects in this anime, the author is misogynistic, every girl in this anime is boneless, pair of tits that chase guys.
no strong woman lead in this manga, and not any developed woman characters.

But hey its the same guy the wrote Angel cop - a manga about American-Jewish bankers wants to destroy Japan so i'm not surprised.

3rd and last thing I am gonna talk about - (but its not like the manga have no more flaws than that)
The characters - Gantz have a lot of characters, some are good, some are not.
The characters that are not, are usually typical and boring, just waiting to die.
(but it got a lot of good characters too)
It will be more interesting to have more character developments beside our main character and really a few more. (Also I would like more rounded characters)

Gantz is a great manga, a huge gift for the seinen genre - I didn't read it 3 times because its boring!