As aeh noted, there doesn't seem to be evidence that boys are more likely to be gifted than girls.
Our experience has been that certain types (most?) gifted boys tend to be such absolutely abysmal fits for the typical classroom environment that their parents can either decide they are delinquents-in-the-making/give up or go searching for what is really going on and find out that their child is gifted. As HK pointed out, most girls fit better into classroom environments and thus, while they may be suffering or developing unproductive habits/beliefs about themselves, they are less likely to wreak the sort of havoc described above re: boys.
I would keep trying gifted programs, camps, etc... whatever her interests happen to be, follow those to the levels that might bring gifted female peers there as well. We have seen a fair number of girls at a university's weekend program, but I do think they tend to be more concentrated in the art and writing workshops. Be sure to let her know that, although it may seem she's alone in her giftedness among female peers, they are out there. They are just going to be harder to find. In my own 40 years, I've come across probably a few dozen gifted females with whom I've personally "connected." Most were flying under the radar, or showing up as "overachievers" in a few cases. The latter were not really overachieving, just showing enough giftedness to do well at whatever the program, class, etc.
