As a non-American I used to believe that "guy" referred only to men. Has it always been non-gender specific?

No it became generic and neutral over time. Though some people still use it to refer to men only. Depends on the person.

I learned much of my English from tv shows live Friends and they do this all the time. I thought it was some kind of joke at first.

It’s suppose to but in casual conversation it doesn’t matter

‘It’ covers all

As a non-native English speaker, for some reasons at first I thought it was pronounced /gooy/ 🙈

I identify as fucked up