The story is that she was AMAB, but raised as a girl because of the village's superstitions about male twins. In the earlier games she identified as a boy (but wore feminine clothes anyway) and lives that way for awhile. But then, in the recent game... I'm just going to quote the wiki on this bit

Sometime after the G4 Summit, Goldlewis Dickinson locates Bridget and tries to add Roger to his cryptid collection, but she resists and Goldlewis backs off. When he casually calls her "li'l lady", Bridget corrects him by listessly saying she is a boy. Goldlewis deduces that she is anxious, product of hiding her true self, not having told her parents yet. Conflicted, Bridget seeks Ky—who has admirably gone public about Gears—out, and he encourages her to decide for herself how she wants to live. Her conversations with Ky and Goldlewis lead her to: in one path, think some more about her own happiness; in another, admit to being scared of losing what she has but no longer wanting to lie to herself, and asserts that she is a girl; and finally, find the strength to live as her true self no matter what others think, and Ky invites her to come visit Dizzy sometime.

So when she first leaves the village she tries living as a boy for awhile, but eventually decides its not how she wants to live and realizes that she is in fact a girl. Some chuds say that its a shitty storyline because it means "the grooming was successful" but I think that takes away all her agency in the situation. She's deciding for herself that she is, after all that, a girl. It is kinda funny though because it could be semi-joked that she's amab but also a cis girl because of her origin story. I'd stay she still functions as a trans girl within the greater world though, that would really only apply in her home village.