Gender is more properly applied to linguistics.
Gender is a social construct.
Sex is biological - chromosomal.
That's fairly modern. Prior to the 1970s, the word sex was used for both, almost exclusively. At some point, the word sex got "sexualized," to where people started associating it with the act of intercourse, and thus felt uncomfortable using it to refer to the division between male and female, so people started substituting the word gender (borrowed from language studies) for sex, unless they were using sex to refer specifically to intercourse.
