The granting of a GRC to a male is a huge statement of trust that you will 'live as a woman' if you then commit a sexual offence against a woman/child (in a male way ie using your male genitalia) should that GRC then be revoked? Part of prison is a deprivation of liberty...
The current prison policy is bonkers, issuing GRCs is crazy but just like you can lose yr driving licence, might GRCs be taken away for bad behaviour?