It doesn't help that "gender" is used to mean (at least) two entirely separate things.
"Gender" - old fashioned social sciences meaning. A technical term of art which roughly corresponds to the lay understanding of sexual stereotype. The set of socially constructed (and culturally relative) norms of behaviour, dress, occupation, pastimes, interests which an individual society deems appropriate to members of each sex.
"Gender" - newer meaning within the trans community. An internal sense of one's own woman-ness, man-ness (sorry, trying to decouple this from "femininity" and "masculinity" which are gender in the social sciences sense) or non-binary-ness.
Most old school feminists like me agree that gender in the social sciences sense is a useful theoretical tool to analyse the political oppression of women - but it isn't rooted in nature.
Many old school feminists, myself included are "gender atheists" regarding gender in the new trans-community sense. We see it as akin to a religious believer's belief in an immortal soul. We can see what they're trying to say, we get that it's deeply important to them to conceptualise the world in this way, we just don't experience it or believe it ourselves.
But biological sex just is. It's part of the natural world. It's a contingent biological fact about me which has no bearing on my personhood, moral worth, political value, intelligence or anything else. It is of no more importance regarding these things than my eye colour. It is however, a contingent biological fact which has been weaponised against me by sexism and misogyny (just as the contingent biological fact of skin colour is weaponised against the BAME community by the weight of colonial history, slavery and racism).