Gender is the stereotypes associated with a person’s sex.
Gender critical people say that a woman is an adult female human, regardless of whether she is a pink-loving fashionista or a petrol-head football fan. Similarly, gender critical people say that a man is an adult male human, regardless of whether he is a pink-loving fashionista or a petrol-head football fan.
Genderist people say that these stereotypes are more important than biology in defining what a person is.
Gender critical people disagree.
TLDR:
When it comes to how people interact,
Genderists: gender and your own opinion of yourself are more important than biology.
Gender Criticals: biology - ie sex - is what matters and gender is, at best, personality, at worst, imposed stereotypes.