I feel like the title is really not necessary, because why should you add the "girl" part to it? Guys don't say "gamer boy". It shouldn't matter the gender. If you play games, you're just a "gamer". Plus, I feel like some girls call themselves "gamer girl" just so they can feel special or quirky.
Most Helpful Guy
The term gamer-girl is often specifically used for girls who try to gain extra attention for doing things in typical male areas, like in this case gaming.
It's most often used with a derogatory association due to those attention-seekers. Those are the kinds who ask for extras just for being a girl instead of just playing and working as a team and thus are simply annoying and disrupting.
Most guys who aren't horny teenagers only capable of thinking with their dicks, know that and will therefor shun and avoid these kinds. It's not because they are girls, neither because the guys are misogynists, sexists or whatever term - but simply because these girls are attention whores.2