Well, it's a problem that many people suffer from that is completely unrelated to gaming: low self-esteem. These girls want to be part of what they see as an exclusive boy's club. It is true that women are currently a minority in the gaming industry, which is why gaming men find girls who play video games more desirable. Girls who feel the need to be loved try to sneak their way into this "boy's club" using video games as a pretense to say that they're a "tomboy" or "just one of the guys." But the reality is that it is not really for the "equality" that they constantly complain is absent, but for the feeling of superiority over other girls coupled with the adoration of the male gaming community.
"Gamer girls" hate on each other because they feel threatened by each other. Another female in this "boy's club" diffuses the overall attention that a girl will get. Therefore a "gamer girl" will rip into another female gamer to protect her status as the most important girl in this male-dominated social circle. This is not unique to the video game industry, I've seen it happen in other predominantly male territories such as tech schools and the local rock climbing gym I used to frequent.