I think she /was/ an ordinary girl. Kind of like Rose and the bad wolf thing, I suppose. Or Amy dreaming the world back into existence. She becomes much more mythic once she jumps into the time stream (which now never happened, right? except that it did). And apparently each version of her is well suited to where she needs to be- I mean, it's not like the earliest version of her we saw just wandered on to Gallifrey. She belonged there. So I suppose that version of her *was* Gallifreyan, by some measure?