You may want to start by asking them about what experience they have with treating trans folk. If they have none, it may be more prudent for you to seek a Dr. who does.
Regardless of your current GP’s attitudes about trans folk and treating them, they may not be your best choice for actually being treated if they don’t have the knowledge to do it effectively.
A properly educated and motivated Dr. should tell you the same. It can be considered unethical for them to treat you for a condition they know nothing about.