As an ex-GDST girl this was fantastic to see. GDST goes where Fawcett, Women's Aid and all the rest fear to tread! I'll be writing to them in support.
It was inevitable that they (and other single sex schools) would have to take an explicit position on this.
Firstly, their business model relies on parents who want single sex education for their daughters. If they deviate from that they are toast. So it is excellent that by this policy statement they have been really, really clear that the consequence of admitting any boys, even ones who identify as girls, is that your single sex school is not single sex anymore. Once you admit one boy you have to admit all the ones who pass the entrance exam.
Secondly, if they became mixed sex, it would almost certainly put them and their member schools in breach of charities law as they would no longer be centering their founding charitable objectives. And it would be totally impractical as the schools do not have the facilities or space for mixed sex safeguarding (separate locker rooms etc).
So in the face of prospective parents (and quite possibly some staff) who are committed gender ideologues and would seek to erode single sex provision, they had to have a clear policy on this, otherwise you would get people continually testing and weakening the boundaries.
It's a bit disappointing that it's not come out in the form of a full throated I AM WOMAN HEAR ME ROAR defence of girls' single sex education, but, actually, we don't need that. All we need is that organisations be grown-ups, calmly defend their boundaries, and centre girls. That is what GDST have done, and that is a radical act (I'm sorry to say) in itself.