Absolute, it was actually US psychiatrist Robert Stoller in the 1960s who popularized "gender" in humans as something different from sex, though he thought "gender" was associated with sex and believed it arose from sex, meaning has a biological basis. John Money seized on the idea, but changed it, postulating that everyone has a "gender identity" that is entirely constructed and therefore is separate to sex and can be opposite to one's sex. Money further had the idea that a person's sense of their own sex and their "gender identity" was solely determined by one's gonads and the configuration of one's external genitals and urinary anatomy. This led him to believe that if the penis and testicles of a baby boy were removed, and the boy's parents raised him "as a girl," and he was put on exogenous estrogen at puberty, he would develop a "female gender identity." A theory he tried - but failed - to prove to disastrous results by his cruel experiment on, and twisted treatment and sex abuse of, David Reimer and his brother, both of whom would take their own lives in their 30s.

Stoller published his book Sex and Gender: The Development of Masculinity and Femininity in 1968, but he had written and lectured on the ideas contained in it earlier.

Also, it's important to note that Stoller's ideas about "gender" came from his work treating and studying people who in the 1960s were known as transsexuals.

Feminist scholars and writers in the 1960s, 70s and most of the 80s generally didn't speak much about "gender." Or about gender at all.

Perhaps some who exclusively wrote for academic audiences did, but the majority of scholars as well as people who wrote about "women's issues" and feminism for wider audiences & the general public talked about sex stereotypes, sex roles, sexism, sex norms, sexist expectations, sex-based oppression, sex discrimination and so on. The widespread use of the term "gender" as a euphemism for sex and to mean sex stereotypes & sex roles is pretty recent, and not the doing of feminists.