I'm sorry, but that's not true. "Gender" is a word with it's root "gen" meaning origin. Gender was a word that was developed to describe the social role associated with a sex, as it was recognized that there were people who did not always conform to the expected social performance (women who passed as men in order to get men's jobs, for one ubiquitous example.)

The word 'sex' is the correct one to describe reproductive category. Gender started to be used in place of the word 'sex' because Americans are squeamish about anything suggesting sexuality.