The Shinjuku massacre was a killing spree committed by Shion Izumi, taking place on Shinjuku street. Shion does so while disguised as a black man wearing a large hoodie, carrying a massive array of weapons in a bag strapped to his shoulder. Shion does so in order to gain re-entry into the Gantz game.
Preparation for the Massacre Edit
Gantz sends a small black ball to Shion when he senses his return to Tokyo. Through this he communicates with him. He informs him that he is upset that he has been getting so many inadequate hunters, and that he would like a stronger team.
He speaks to Kei Kurono beforehand, warning him about the massacre he is about to commit and telling him to kill him if he wants it to stop. Kurono is unable to do so, however.
The Massacre Edit
On the day of the massacre, Shion disguises himself as a black male wearing a large hoodie, creating somewhat the illusion that he is a foreigner. On his way to Shinjuku street by train, he meets a young man who asks him whether or not he plays sports. Shion then tells him to follow him, promising he will show him something cool.
When exiting the train, Shion and the young man enter the street of Shinjuku at 10:00 am. Shion suddenly pulls a handgun from his bag, the young man asked if it was an airsoft gun. Meanwhile, Izumi was seen heavy breathing before finally shooting his first victim, terrifying the young man and everyone around him.
Shion then proceeds to shoot everyone around him and begins hunting down the fleeing civilians using a sub-machine gun from his bag, killing the young man in the process.
During the massacre, Shion himself is close to being killed several times, firstly when he is approached by Daizaemon Kaze. Carrying the body of a deceased man to protect himself from Shion's bullets, Daizaemon manages to reach and overpower Shion without much effort. As the crowd cheers, Kaze almost manages to strangle Shion, only to be shot in legs and killed just before being victorious. Moments later, Shion is faced by Hiroto Sakurai and Kenzo Sakata who used their psychic powers to stop the incoming bullets.
While Shion manages to shoot and kill Kenzo before he is able to get close, Shion is almost killed by Hiroto's ability to destroy internal organs using his powers and is almost dropped to his death, only managing to save his life by shooting Hiroto in the head before he reaches too great a height.
The Shinjuku massacre caused the death of more than 150 people. 22 ended up in room along side Kurono and Izumi. 
The Gantz manual says 372 were dead. The TV report about 387 deads and injuries in chapter 165, page 3.
Once inside the Gantz room, a message is displayed: "Everyone's been so weak lately... That's why I wanted to bring the strongest people here..." 
- In chapter 124, page 10, Kurono calls Tae. His phone says it is 15:53, even though the Shinjuku massacre takes place at 10:00 am.