That is so cool that John Lithgow portrayed her with that much love. He is clearly quite intelligent and open-minded.
When I saw that movie in the '80s, I clearly recognized that part of me, but thought that much like Lithgow, I could never, ever resemble a woman in a million years. I thought, much like Lithgow, that I would just look like a shaven linebacker in a dress and/or something of caricature of a transwoman. What I had not considered, was that Lithgow was not on HRT nor had undergone electrolysis, and what a difference those things actually make. Sadly, watching Garp was something that drove yet another nail into my closet door, or put another brick in my wall, or <insert cheesy metaphor here>.
At any rate, John Lithgow is a fantastic actor and an all-around great guy. I would also like to see more transgender actresses in transgender (or cis!) roles.