Didn't read through all the posts because I'm too lazy so I don't know if anyone has pointed this out or not: There is no reason to think girls wouldn't like Gantz because of the violence and gore. There are tons of shoujo manga out there filled with gore. The only possible thing that would deter girls from liking Gantz is the fanservice, in my opinion. But clearly there are plenty of girls (like myself) who don't mind the fanservice at all. I grew up reading American comics, so huge boobs and ridiculously proportioned female bodies barely even make me blink. XD I also love Berserk, Tenjoh Tenge, and plenty of other series that guys seem to think girls don't like.
I think the problem is that a lot of girls simply don't like to post in forums like these. Look at my join date and then at my post count. I joined Stoptazmo for the purpose of downloading more of Gantz after omanga stopped releasing it. But I've only posted a few times because I'm just not comfortable posting. I imagine there are other girls who feel this way. And even so, there are tons of girls downloading Gantz from other sites who never even see this forum.
Also, I don't think reading manga like Gantz is a sign of being a tomboy. I have just as much love for shoujo manga as I do for shounen manga. I just like good stories, and I think there are plenty to be found in all categories of manga (I wish more boys would try out some shoujo! There are some gems there, guys, trust me).