Within the school uniform. Mine is 5 so hardly the best person to decide with regards to what's appropriate to wear for the given day, she's wear the summer dress complete with white socks in a monsoon given the choice.
5/6 year olds with "gender issues" are more than likely mirroring their parent's problems would you not suspect?
I feel choice is important for kids, helps them develop into better human beings than just being told what to do all the time.
I am going to guess that for this kid the parents are fairly comfortable about gender nonconformity if they have gone out and got the girls school uniform for their boy.
However strict parents can also be a problem. If a boy doesn't really like what he is constantly told are boy things then he is possibly going to question his gender. He may also question his sexuality. Same for a girl, though girls seem to have a bit more leeway in that respect (they can like superheroes and wear trousers and no one cares).