From the World Health Organization:
"Gender refers to the characteristics of women, men, girls and boys that are socially constructed. This includes norms, behaviours and roles associated with being a woman, man, girl or boy, as well as relationships with each other. As a social construct, gender varies from society to society and can change over time."
Don't know who you think "the guy who proposed. . ." is, but it's scientific/sociological consensus.
And the hormonal constructs you're referring to develop complete independently of the sexual organs and at a variety of levels (still a spectrum).
Trauma can cause many psychological developments and I suppose that could be one of them, but it is certainly not an exclusive factor.