Gender not existing except in grammar?
Up to a point, Lord Copper.
If used thoughtfully and with nuance, it's a useful term. That is, if it's used almost as an adjective: "gender roles and stereotypes" for example.
It was useful in the 1970s in my field of work as a way of indicating that there's a separation between biological sex and the socially and culturally constructed roles we expect humans to play, mostly dependent on their sex.
These roles are socially & historically specific.
Feminists needed to show how a person's sex did not necessarily determine or limit what they could do, study, work at, or be like. Until very recently, feminists were trying to achieve simple equality with men: equal pay, maternity leave, no rape in marriage, equal splits on divorce, equal pensions - that sort of thing.
So it was essential to have two words top separate out sex as a matter of biology, and people's social existence. To stop the nonsense such as "Women have babies, therefore their brains are weaker" sort of stuff.
Nowadays we might be better calling gender roles and stereotypes "sex-based stereotypes."
But "gender" has been a useful word in feminist thinking and action.