Actually that's a common misconception - while gender dysphoria is listed in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) - it's not actually a mental disorder (similar to how the DSM includes physiological and environmental issues like Insomnia or Social Exclusion) - main reason it's there is for admin purposes and to facilitate treatment access.

However, a condition like body dysmorphia (think Anorexia Nervosa) is considered a mental disorder because the issue is the mind incorrectly perceiving the body - therefore it can be treated using psychotherapy which enables the mind to correctly perceive the body and prevent harm.

People who experience gender dysphoria on the other hand - actually correctly perceive their body (that's where the distress comes from) so psychotherapy doesn't work to alleviate this - as you can't therapy away an accurate perception (think gay conversion therapy)

Which is why after many decades of research the only treatment that's been found to work is aligning the body with the mind - as at that point the mind continues perceiving the body correctly but this time it's congruent with it's mental model which alleviates the distress.

Hope this helps :)