“Homosexual” has a clear, specific, technical meaning. “Gay” has several meanings, only one of which relates to sexual preference.
All homosexuals are homosexual.
Some homosexuals are gay (depending on which meaning of the term one is considering or intending).
“Some homosexuals are gay (depending on which meaning of the term one is considering or intending).”
If you are a homosexual you say hello. If you are gay you say hellloooooooo!