Gender is the choices we assign based on biology. Woman born male is someone making the choices of a woman born with a male sex. She is not female, or making female choices, simply choices that we associate with females, and there is a difference. Female choices are menstruation, for example. Gender is transient, in the mind.
Sex is permanent, in the body.
We don't understand sex completely. Some is in the genes, some in the hormones, some in brain anatomy. The differences between the sexes at a deep level are not understood, but we think we know enough by looking at someone's crotch. That's why we can't talk about genetic females, but only anatomical females . We don't look at their genes, only their crotch.