The answer is becoming increasingly elusive, because whatever you think it might be, or include, there are always some people who identify as trans but don't do it, so they obviously consider it an optional extra.
Examples might include:
Having gender surgery and/or other body and facial modification
Taking feminising/masculinising medication
Wearing typically opposite-sex clothing
Getting rid of your beard
Expecting to use opposite-sex facilities
Insisting other people use opposite-sex pronouns for you
Bothering to get a GRC
Refraining from behaving like a stereotypical member of your birth sex
And now- refraining from living the reproductive life of your birth sex
So if all these things are optional and unnecessary for beginning to live "as a man" or "as a woman", what exactly is necessary? What is left? What do you have to do/not do? Clearly the GRC awarders have some sort of standard that must be met (or they wouldn't have said no to this person), but what definition are the people who rejected their decision using?
Ironically, when woman and man are defined as having a body organised round the production (successful or otherwise) of large or small gametes respectively, the problem disappears, because all the sorts of things listed above are truly optional. You don't have to look or behave or dress in any particular way, or have a particular kind of character or interests in order to be a member of your sex; you don't have to bear or beget children or even be capable of doing so; you don't even have to actively "identify" as a man or a woman, whatever that means.