Just last month, the Medical University of Vienna issued a press release announcing that "the very personal gender identity of every human being is reflected and verifiable in the cross-links between brain regions," with distinctions specifically in what's known as brain "white matter." This discovery was made via a study led by Georg S. Kanz of the University Clinic for Psychiatry and Psychotherapy. Researchers used an MRI to measure the diffusion of particles across brain matter in 23 trans men, 21 trans women, 23 cis women, and 22 cis men. They found that, "while the biological gender is usually manifested in the physical appearance, the individual gender identity is not immediately discernible and primarily established in the psyche of a human being." This is concrete evidence that gender identity exists on a scale.
You can read the full press release here.