The Shinjuku massacre took place on Shinjuku street, when Shion Izumi decided to go on a killing spree. During the massacre Izumi disguising himself as a Black Man wearing a big hoodie, and carried his weapons in a bag swept around his torso.
Preparation for the Massacre Edit
Gantz sent a small black ball to Izumi when he sensed he had returned to Tokyo. Through this he communicated with him. He informed him that he was upset he had been getting so many lame hunters lately, and he wanted a strong team. He wanted him to bring people over at a set day.
Instead of going to where martial artists, soldiers, athletes or others potentially valuable hunters were at, to bring them over at the appropriate day, Izumi decided to just shoot up people at random on a crowded street. It is possible he just didn't think of the best way to get strong hunters, or as seen by how he doesn't explain things to people, try to help them on his first mission back, and is quite surprised so many had survived when it was over, that he did not want competition, and figured just random people would do.
The Massacre Edit
On the day of the massacre, Izumi disguised himself as a black male wearing a large hoodie, creating somewhat the illusion that he was a foreigner. On his way to Shinjuku street by train, he met a young man who asked him whether or not he played sports. Izuni then told him to follow him, promising he would show him something cool.
When exiting the train, Izumi and the young man entered the street of Shinjuku at 10:00am. With Izumi suddenly pulling 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.
Izumi then proceeded to shoot everyone around him, and started hunting down the fleeing civilians. Using sub-machine guns from his bag, he shot the young man and proceeded to hunt down the remaining citizens.
During the massacre, Izumi himself got close to getting killed several times. First when he was approached by Daizaemon Kaze ; carrying carrying a body of a deceased man to protect himself from Izumi's bullets, Kaze managed to reach and overpowering Izumi without much effort. As the crowd cheered, Kaze almost managed to strangle Izumi, only to get shot in legs and killed just before being victorious. Moments later, Izumi was faced by Hiroto Sakurai and Kenzo Sakata who used their psychic powers to stop the incoming bullets.
While managing to too shoot and kill Sakata before he got close, Izumi almost died from Sakurai's abilities to destroy organs. Izumi was almost nearly dropped to his death, and only managed to save his life by shooting in the head Sakurai from a distance.
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..." 
Since Gantz had random people coming in all the time, there is no possible reason he'd send Izumi to do more random gathering.
In chapter 124, page 10, Kurono calls Tae, and his phone says, its 15:53, even though the Shinjuku massacre started at 10:00am.