One starts with the official sites:

http://www.gantz.net/ (Site in Jap)

http://www.fujitv.co.jp/GANTZ/index2.html (Site of the chain TV where Gantz was diffused, site in Jap)

http://yj.shueisha.co.jp/ (site of Young Jump where the manga is published, site in Jap and Flash)

http://www.konami.jp/gs/game/gantz/ (Site of Konami, the editor of the video game)

http://www.gonzo.co.jp/english/titles/0405.html (Site of the studio which produced animates it, in English)

http://www.advfilms.com/titles/gantz/ (Site of US editor DVD)

http://www.gantz.com.au/ (Site of editor DVD Austalien, in English donc^^)

http://www.geocities.co.jp/Technopolis-Jupiter/2367/  (this is official Nishi site made by Oku him self)

I nevertheless put the site of that to you which will publish Gantz in France:

http://www.asianstar.fr/

sites that sell the manga:

http://stores.ebay.com/Jade-Dragon-Formosa-Tea-and-Manga/?refid=store (here you can buy the Chinese vere of Gantz and in a cheap price)

Traslator

http://shocwave.caticus.com (Current English translaslation group working on gantz)

http://www.nsgmanga.com/ (Current spanish translaslation group working on gantz)

nonofficial sites:
In French:

http://perso.wanadoo.fr/denverman/partgantz/gantz.htm (not of update since several months)

http://gantzotaku.free.fr/ (great site + we help each othe make our sites better)

In English:

http://www.filefarmer.com/gantzball/home.html 

http://www.livejournal.com/community/gantz/

http://groups.yahoo.com/group/Gantz_Is_Real/message/5

http://gantzrp.proboards58.com/ (a site for Gantz DnD theme game

http://en.wikipedia.org/wiki/Gantz (a online encyclopedia entry)

http://www.stoptazmo.com/gantz (direct download on all Gantz chapters)

http://www.stoptazmo.com/viewforum.php?f=4 (a part of a forum devoted to Gantz)

In Russian (yes yes, in russe^^):

http://www.gantz.ru/news.php

In Chinese:
http://www.freepgs.com/gantz/home.php (Hong-Kongais site, not on that exists this mot^^)

In Japanese:
http://www.geocities.jp/nagai_daisuke/gantz/

http://gantzlabo-nexststory.ameblo.jp/ (Site which exposes several theories on the dernies left chapters)

http://hen.risu.com/ (a site which treats of a little warp works of Oku)

http://gyooooon.fruitblog.net/ (a blog devoted partly to Gantz)

http://www.geocities.jp/ohda_chugaku/

http://yteikoku.com/modules/freenovel/spl_diary.cgi?action=showlast&cat=14&txtnumber=log

http://www.geocities.jp/gantz11takasan18/topgan.html (Yaoi Site)

http://www.geocities.jp/btsbk765/gantz/home.html (Some Fan Art)

http://free.from.jp/musashi/?k =

http://www9.ocn.ne.jp/~barukou/top.htm

http://www.geocities.co.jp/AnimeComic/1007/

http://www.geocities.jp/gantz_fan/ (a rather complete site)

http://www13.ocn.ne.jp/~n-ask/gantz.htm (a Little Fan Art)

http://www.geocities.co.jp/Stylish/3627/gantz.htm

http://rowen.air-nifty.com/rsm/gantz/index.html (a blog devoted to Gantz)

http://www.MetkuMods.net/ - A site of diferents mod

http://www.Imageshack.us - Site for hosting image(most of my images are been host from there