What is the purpose of a GRC? Why are they needed? I don't see why it's necessary to have legal rights based on something that isn't true.
The GRA (2004, I think) brought in the "legal fiction" of a change of sex, so that people could stay married. It was a piece of legislation before equal marriage.
I think it's fair enough that if someone is so ill/distressed by their sex that one solution is to seek to mitigate the effects by living as/presenting as the other sex. And then to be able to have legal rights against discrimination on the basis of being transgender.
The Equalities Act still allows for birth sex to be a factor in some cases - proportionate means to a legitimate end. Single sex safe spaces for women, such as refuges and rape crisis centres completely fit this category.
But the whole thing has become so muddied by the push for self-identification (I'm a woman because I say I am) and many organisational & institutional policies which assume self-id. Unlawfully in some cases eg this one.