No science to see here, so it's just his belief, so it's bigotry, so Google needed to fire him, because their policy that forbids harmful gender stereotypes, well, forbade him saying what he said.
Fired Google Engineer: Company Executives Smeared Me
He says the company policy stifles free expression and frank discussion of the issue.
Um, yup, exactly the point of PC prohibitions on speech.
Still, don't know that any court has ever held that the First Amendment protects people from being fired.