... About bias... I interpreted it to mean is that the questions on that version of the CoGAT were skewed so as to be answered more correctly by boys than girls.
As in... the sets of tasks to perform, while the same for boys and girls, were inherently more appealing to "stereotypical" boys and/or especially off-putting for "stereotypical" girls?
Personally, I would explore this topic with the director of gifted ed who originally mentioned the test's gender bias to you, so you may clarify this understanding/interpretation.
... 75/25 split was not just for one year's data, but a trend over time.
Combining this with "across the district" as mentioned in the original post... if this 75/25 split is the trend for the relatively same population of students as they advance through the grades (about 80% of students being the same gifted math class rising one grade each year, while about 20% of the gifted math population is replaced annually as identified incoming 3rd graders replace 8th graders)... the relative stability of the statistical split may be expected from year to year.
What points to me that something is amiss is that neighboring districts that have a similar population of people (and thus I would expect a similar profile of students) have more of an equal number of boys and girls in their gifted math class.
You may wish to make an inquiry of the gifted programs at the neighboring districts you are referring to, and learn whether they are utilizing quotas, as well as what their identification/qualification criteria are. It is often much better to be guided by facts than a series of hunches.
Our district will not use outside testing to determine placement. However, we were able to use my DD10's EXPLORE test results to get her retested on the CoGAT to see if she would now qualify for the gifted math. They did not have the MAP test when my daughter was in 2nd grade and testing for gifted math, so I suspect they gave even more weight to the CoGAT then.
You may wish to inquire of your director of gifted ed to learn whether the CogAT was weighed more heavily than 60% under the prior identification criteria. Again, best to have the facts as you prepare for any advocacy.
They also said that they see the trend as girls get older they tend to drop out of the gifted math classes which is probably partially due to peer pressure as someone else stated.
A survey of the parent(s) of the child leaving the gifted math class, and of the child leaving the gifted math class may help gather facts about the gifted math class and the reasons why students choose to leave it.
That is definitely another problem to tackle, but I think it is separate than the initial identifying in 2nd grade.
Good luck with this... there is some good detective work and fact-gathering to be done here. Many may benefit.