The idea of repealing the GRA still sits outside the Overton window, but the window has moved in that direction for many reasons to the extent that repeal no longer considered an extreme position.
It is not currently politically possible and a change to a Labour government won't help. (Indeed on a free vote there wouldn't be anything like a majority of all MPs in favour right now.). However, voices calling for repeal publicly are now important and are no longer outside the bounds of 'acceptability'. The very act of doing so will further cement and normalise the idea.
The FWS2 ruling, whilst a disaster in some ways, will help in others including strengthening the idea of GRA repeal. Never waste a crisis.
I have concerns over the Sex Matters approach and its possible unintended consequences, whilst at the same time understanding that, in terms of politics being the art of the possible, their approach has a greater chance of success in the short term. But on balance I think it is the wrong approach.
Until we remove the legal untruths that the GRA facilitates we will always be one step behind. At the same time the gender reassignment protected characteristic also needs to go to be replaced by the existing characteristic of belief. After all, if sex immutability is now a protected belief then sex mutability should not have a higher status.