[rktman]

Last I checked, one can’t transform their actual gender. Cross dress and alter their physical being, yes.

[arthurus]

Gender doesn't have anything to do with mentally ill peoples' identity or changes and there are three genders, not two or more than three. Gender is a grammatical term for sorting words into masculine, feminine or neuter cases. English has lost almost all of that while the Latinate languages gender their nouns and change adjectives to agree. German is the extreme of the gendered languages. Gender is a grammatical system along with conjugation and declension.
The proper word which used to be used is "transsexual."