mummy, if it was unclear, I was being sarcastic (to be fair, it's hard to tell these days- some people really have idiotic views).
If you swallow the mantra that trans girls are girls because they say they are girls, we are talking at cross purposes. I don't know what you mean by 'girl' and presumably you don't either, beyond it being 'anyone who says they are a girl'. Trans girls may indeed be girls according to your definition. I am talking about a child of the female biological sex and under that definition, children born male are not a part of it.
My position is the same for adults. Trans women may indeed be women, if your definition of woman is a loose concept of anyone who identifies as it. My definition is adult humans of the female biological sex. No more, no less.
With GG, the main issue is not membership per se, but the absolutely idiotic and blatant lack of safeguarding on overnight trips and the refusal to allow parents to give informed consent as to whether they want their daughters to share sleeping space and showers with a biological male.