GRC = Gender Recognition Certificate.
Its one and only purpose in the UK is to allow the holder to get their birth certificate changed to show their "new" gender.
Once you have one, it is illegal for anyone to ask to see it for any other purpose than that, or to tell anyone else they've seen it, thus outing you.
You have to have been provably living full-time for 2 years to get one - it is not necessary to have had GRS to get one, though it's easier if you have.
It is in no way "proof" of gender. It's only meant for that one purpose, to change the birth certificate. Although, in practice, if you want to change your gender status with the tax office, you need one. It's all a bit badly thought out.