While a GP can provide this, not many GPs feel that they have the necessary knowledge or training to “diagnose.” Much more work is needed in this area.
Other people who can help are local endocrinologists, GPs with a special interest or other gender specialists.
You can also get a diagnosis through a Gender Identity Clinic (GIC) but the waiting lists can be very long.
Counsellors and psychologists can all give you a diagnosis if that is what you require to achieve your goals.