Several people dealing with this at the moment. My DD had come to a very firm conclusion during a period of severe depression, before she told us, which has now set like concrete, unfortunately. If your daughter is up for discussion (mine isn't) could you approach this as one possibility that deserves to be explored, but that there are other possibilities too? There's a good chapter in a book called Aspie Girls which sets out the difficulty some girls with ASD have with gender. But I think the best thing would be a skilled psychologist with experience of ASD - good luck, I don't think there are any easy solutions.