The way to go on this is to chip at it. No party will go straight to repeal.
Putting aside for a sec that no man is a woman whether he has a piece of paper or not, one of the problems the GRA has caused is the privacy clause.
The privacy clause aspect of the GRA has had the real life effect of enabling any man to take advantage of it. This needs to stop and businesses/orgs etc. should be able to ask for proof of this legal sex/gender certificate.
Obviously women and girls will still have to contend with the problem of some men in their spaces and it will be even more of an issue if Labour make getting one simpler.
However if we view it as building case for repeal step by step then the first stage as above is removing the chancers and opportunists. We all know this won't totally resolve the issues because out of the men with a GRC there will still be those that pose a danger and cause issues for women and girls.
So then we go back again to the politicians and make our case, with evidence (and there will be evidence!) that these men with GRCs are no safer, no more appropriate to be in spaces with women and girls than the other men that were excluded.
Eventually we will box them in so there's no recourse other than repeal. It will be a bloody long slog though and we are probably still years away from this.