Gender is a linguistic term (e.g. in French where words may be masculine or feminism) rather than a biological one. Leftists like this term because gender may be "constructed" (i.e. made to mean whatever you wish, which is where the notion of "transgendered" comes from), whereas sex is unavoidably biological.
Words matter, so conservatives who want to fight the good fight and win should avoid misusing the word gender when they really mean sex.
Thank you, words do matter!