Any organisation now asking for gender rather than sex in any capacity is now on very dodgy ground. They can ask for gender as well as sex but a failure to ask for sex whilst asking for gender is a complete no no.
This will be one of the things that will work out in the wash.
Lying about your sex under certain circumstances will also become a massive no no (eg if taking part in a medical study or as part of a safeguarding disclosure) even if you have a GRC because of the impact on others. It will become a red flag if someone is being treated in hospital and it's picked up that they haven't disclosed their sex if relevant.
This is how it should be. Activists can grumble all they fucking like. The entire point of our human rights and equality rights legislation is to balance, compromise and protect the interests of EVERYONE. Shocking huh?
This is where narcs are going to become unstuck because it's not all about them. Everyone else will just get on with it. Its not a trans issue - it's a selfish narcissist issue.