Gender Dysphoria is considered a Mental Disorders and is in the DSM-5 from the American Psychiatric Association.
Gender dysphoria is feeling uncomfortable in your gender-at-birth. Therefore, transitioning or rejecting the concept of the gender isn't the disease — it's a cure for the disease.
Of course, that transition also usually leads to other types of discomforts caused by other people not respecting their decision, but hey, it's not trans people's fault other people are dickheads.