I thought it was John Money who first proposed the idea that a person's "gender" was separate from their sex, and attempted to prove it by performing vaginoplasty on a boy after a botched circumcision and having the parents raise him as a girl.

This didn't work out too well, and the parents eventually had to tell him what happened. He went on the Phil Donahue show as an adult to talk about it, which you can find on YouTube.

His brother, and then later he committed suicide.

Where Money and modern gender ideologues differ is that they believe gender identity is innate, so Money's approach wouldn't have worked if the kid's identity wasn't inherently female. But nevertheless queer theorists held on to the idea that sex and gender are completely separate.

I'm sure these ideas predate Money, but some were more influential than others.