A very seasoned administrator in a local gifted school tells me that the number of boy applicants for their sought after gifted program far outnumbers the number of girl applicants. They require an IQ score of 135+ to be eligible for the program. She says that they drop many gifted boys from the applicant list and select girls who do not reach that cutoff just because they need enough girl students to keep a 50/50 ratio of boys and girls in their school. When I asked her why this was the case, she said that around 1st-3rd grades, the very gifted boys who are severely under challenged (as is the case in many of the local schools in my area) start showing restlessness, inability to control their impulsiveness, inability to cope with boredom etc which show up as "behavior issues" in the classroom. So, the parents of these boys come to the conclusion that gifted programs are necessary far earlier than the parents of gifted girls. According to this administrator, most HG/PG girls learn to manage their behavior well in any classroom setting (early maturity?) and read a lot to compensate for the lack of challenging education and have much more self-control in the typical classroom. She said that many parents of gifted girls realize that their child needed more challenges much later because they do not exhibit unhappiness about the school situation often.
This is certainly hearsay and it sort of matches my own childhood experiences and so, I thought I would post it here. None of it is scientific, just the observations of a person who has held a particular job dealing with gifted elementary kids for 30 years or so.

OP: statistically, there are as many smart girls as boys - if you asked my DH, he would say that the girls we meet at places where gifted kids congregate are more gifted and thoughtful than my DS. My son is part of a competitive chess program and there are more girls than boys at the younger ages. There is a math circle that my son attends and there are more girls than boys in the elementary circle. I attended a presentation for the local round of the Intel Science Fair and the local Science and Tech Championships and there were a lot of girls presenting a lot of very advanced science and technology projects. Just thought that you might want to try these options locally to see if your DD can meet more girls like her.