[Shulamith] Firestone doesn’t use the term ‘gender’, but she does differentiate between the biological markers of sex and what she calls ‘the sex distinction’ ... After the revolution there will still be ‘genital differences between human beings’, but they will ‘no longer matter culturally’.
...
Beauvoir didn’t use the word ‘gender’ either.
the meaning of ‘gender’ which depends on an explicit or implicit contrast with biological sex was first used by academics in social science disciplines like anthropology, sociology and psychology
the ‘identity’ sense of ‘gender’ ... first appears in the 1950s, in texts dealing with the clinical treatment of what were then called ‘hermaphrodites’ (i.e., people with intersex conditions) and ‘transsexuals’. It isn’t entirely clear whether this medical usage developed in parallel with the social science usage or directly from it, but in any case the clinicians soon began to produce a distinctive body of knowledge, which included proposals about the definition of ‘gender’.
Robert Stoller ... is often credited with introducing the term ‘gender identity’ ... He believed there was a biological basis for what he called ‘core gender identity’—defined as an innate sense of being male or female which is normally fixed by the second year of life—but he also wrote extensively about the influence of nurture.
John Money, the psychologist who founded the Gender Identity Clinic at Johns Hopkins in Baltimore ... was an influential proponent of the view that gender is learned rather than innate: his clinical observations showed, he claimed, that children acquire the gender they’re raised in, even when it’s incongruent with their natal sex. ... In a 1955 research report, Money and two of his colleagues explained their concept of ‘gender role’, which they defined as ... an internal characteristic of individuals, ‘disclosed’ in their behaviour and what they say about themselves. The missing element of the current meaning is the idea that gender isn’t a binary division: this early definition acknowledges only two categories (‘boy or man, girl or woman’). Stoller, too, assumed that a person’s ‘core gender identity’ must be either male or female. The more recent emergence of alternative categories (including ‘nonbinary’ and ‘genderfluid’ identities) may reflect the influence of queer theory; but in all other respects, arguably, today’s understanding of gender as a form of identity owes more to the medical model elaborated by people like Money and Stoller.