a) "Men who self identify as women" are also known as trans women or, even simpler, women. Since GG is for "all girls" (quote from their website) that includes ALL WOMEN.
"Men who self identify as women" are not known as women, except to a teeny-tiny minority. The rest of us known them as transwomen, trans-identified men, or more plainly, as men.
Incidentally, my friend calls her hens her "girls". Should GG accept hens as members, do you think?
b). It is also well documented that trans women do not pose a threat to cis women in women-only spaces.
Bollocks. It is well-known that they do pose a risk, and also that risk is not the only factor - we can have women-only spaces for privacy, for dignity, or just because we want to.
c) If you are going to say "What if they are men and just want to look at little girls in a sexual way?" then should we prohibit lesbians or bisexual women from volunteering
Never in my long life has a lesbian or a bisexual woman verbally abused me, sexually harassed me, or posed any other kind of threat to me.