Specifically, I like to fund things where it seems to me that the case will have positive effects whether it is won or lost. This is one of those. It's well worth reading the Statement of Facts and Grounds - this makes explicit a lot of the things that we all know, but that cause TRAs and #bekind folks to froth at the mouth and call us bigots. Especially, it lays out the uselessness of making a distinction between GRC holders and non-GRC holders when you can't ask to see a GRC; the ease of getting a GRC while still having a functioning penis; the dual problems arising whether a man with a GRC is, or is not, mistaken for a women; the lack of any evidence that men with GRCs are less prone to sexual violence than other men; and the sexual gratification grounds underlying some men's choice to transition, including, to obtain a GRC. It connects these things to the practical working of a policy. The fact that this policy relates directly to sexual assault and the fear of it (whereas a lot of the successful cases so far have concerned fully-clothed matters of discrimination and harrassment) makes it an important addition, I think.
This case has no direct applicability to SSEs, but I think it does have (win or lose) the potential for indirect applicability. As others have said, the SSEs are voluntary - we have no legal means to force any organisation to apply a SSE. What we do have, as customers etc., is the ability to put pressure on organisations to apply them. But when we write to organisations, mentioning some of the things I listed above from Facts and Grounds, it is easy to be dismissed by people who think "that never happens" and have a picture of trans people as harmless and most-oppressed. Being able to cite things from this case - and the end judgement is likely to be useful for this even if the case is lost - may help.
Finally, concerning repealing the GRA. Personally I think it's not likely that we will ever literally repeal the GRA (the difficulties of what to do about existing GRC holders seem likely to be intractable); the strategy I think is more likely to succeed is getting it gutted, via clarification that what it changes is not sex for all purposes, but (public recognition of) gender. Combine this with gradual recognition that gender should have no effect on anything legally, and we're there: anyone can have a gender recognition certificate if they'd like one, and nobody else need ever care. This case, if we win, expands the range of purposes for which it is clearly agreed that a GRC does not change sex. It's difficult to imagine how, even if we lose this case, it can avoid undermining the GRA - even if, somehow, the policy is deemed lawful, all the stuff about how unworkable it is will be on record, in precise legal language, and that has to help.