I'm not sure how that would work with a GRC? My preference would be that the data would be really comprehensive but completely anonymous so that GRC could also be noted.
I think given where we are currently with the GRA and what the law already says, for statistical purposes, it's not going to be possible to accurately record whether people have a GRC or not (even though you might know if they were standing in front of you).
Since a GRC means people's driving licence, passport, etc will record the non-natal sex, and since the whole point is that officials can't press a person to reveal their gender reassignment, it won't be possible to tell statistically whether sex data has come from a GRC or not.
So my assumption would be that GRCs would be an anomaly. But everyone else's biological sex could be recorded from administrative records, ID, etc (i.e. the same info they use to put them in the right prison in the first place).