In order for trans girls to compete in sports, they are usually tested for testosterone levels. Certainly that's true at the collegiate and Olympic level. There have actually been biological female athletes who have been barred from competition because their natural testosterone levels are too high. There is no evidence that a trans girl has any advantage in competition. And, I defy you to give me a single example of a trans athlete using their gender identity to successfully defeat cis gender athletes. It just doesn't happen.