Sei Sakuraokais based on Angeline Joline's Lara Croft character.
Notice the hair is the same, the black outfit is the same, and both are holding a gun. In the anime, however, due to an art shift, most of Sei's resemblance to Croft was lost, much to the displeasure of fans.
Chiaki is made to look like a character from the film Kill Bill. Gantz even nicknames her Kill Bill. Chiaki also is the first name of the actress (Chiaki Kuriyama) who played the assassain, Gogo Yubari, from Kill Bill Vol. 1. Chiaki Kuriyama even did an interview with the creator, Hiroya Oku, which can be watched here.In the latest story arc, the character Toshio Yazawa was introduced and had a striking resemblence to George Clooney. Strangely enough, the characters that have American actor look alikes don't end just there. A new character who looked oddly like Steve Buscemi was shown as a member of the American team in Chapter 368: Transgression and a Profession of Love.
Sadako is based on Sadako Yamamura, villain character in the horror cult movie "Ringu".
Tanaka Alien is based on the japanese folk singer Seiji Tanaka. It was changed to Suzuki Alien in the anime because of Tanaka's discontent of his portrayal in the manga.
Daizaemon Kaze looks like Kazuya Mishima, one of the main characters in the Tekken series of fighting games. He also shares resemblance to Isshin Kurosaki of Tite Kubo's Bleach.
Masaru Kato is often compared to Kenshiro, protagonist in "Hokuto no Ken". Appearance wise, Kato resembles Kugo Ginjo of the Bleach manga series (though it should be noted that Kato's creation predates Ginjo's appearance in Bleach by at least 10 years).
There is a guy around Chapter 275 who is fighting the giant aliens one on one before being kill by Eeva Gund who looks exactly like MMA fighter Wanderlei Silva though he fights using 2 swords.