I don't mean to say it isn't hard.
Dd told me she thought she was really male in a psych hospital, where the doctors wouldn't let her go home till she disclosed, and I reacted honestly but badly. I was angry, called her a traitor to the cause (not in so many words but that was the gist) and basically displayed my complete lack of belief that female children whose vulva's I observed myself over years of bathing and nappy changing are actually male. I refused to use a male name; I try when I remember to do neutral terms like 'sibling' but often forget.
And you know, she was pretty hysterical on hearing all this. But we managed that outpouring of emotion from both sides.
She knows I am utterly supportive of her being gay, and gender non comforming. I buy her clothes from the men's section. I give her money to get her buzzcuts. She accepts that I will always see her as female, and that I have no problem with females being butch. I think real life acceptance of these things goes a long way to mitigating disgust.
I wonder if refusing to enact cognitive dissonance myself is in fact a point of security for her. I don't know. For all I know she's hanging out to leave home, get hormones, chop her boobs off. But I think we'll be Ok either way, and I didn't have to lie for it to be OK.
So I hope, OP, that I didn't give you the idea it's easy to be GC with teen girls. It's not. It's a tightrope. I more wanted to give you optimism that being honest doesn't have to shut down the relationship.