The more useful question is, what part of the great bulk of Wikipedians who don't self-identify by gender are female?
I second this.
I'd be willing to bet from anecdotal evidence and experience that the greater portion of editors who don't indicate a gender in preferences (I'm one, actually, as it turns out) are female. Women are more likely to have unpublished or unlisted phone numbers, and when they are listed, I know quite a few who use initial pairs rather than a first name.
On Wikipedia, I have also known more female editors than male who have put in username-change requests, specifically from a real or partially real name to something less gender-specific.
Daniel Case