There is guidance on you.gov about how to prove that you have been living in your acquired gender eg a man living as a woman:
Evidence of living in your gender for the last 2 years
You’ll need copies of evidence to show that you’ve been living in your affirmed gender for the last 2 years.
Each piece of evidence should contain at least one of the following that matches up with your affirmed gender:
- a name
- a title, like ‘Mr’ or ‘Miss’
- a gender marker, like ‘male’ or ‘female’
There are no other specific requirements for this evidence, but try to find:
- evidence from different points over the 2 years, with roughly 1 piece of evidence for every 3 months
- at least one piece of evidence from the last 2 or 3 months
- evidence from a variety of different sources
The evidence can come from before you were diagnosed with gender dysphoria.
Examples of what you can send in include good quality photocopies or photographs of:
- your driving licence
- your passport
- etc etc
So it is literally changing your name, title, and sex marker on your ID. There are no other requirements for this aspect of a GRC application (although, I assume there are behaviours that would facilitate getting the GD diagnosis that is also required).
So the correct answer is "living as a woman means changing your identity documents".
HTH