Genderists believe there is a thing called gender that is expressed via clothes, hair etc, that is immutable (despite the fact that conventions around hair and clothing styles a very varied over space and time) and that it is so much more important than sex that people should be encouraged to poison and mutilate their bodies so that they match their chosen clothing.
They may be socially liberal or rigidly illiberal - Iran is a genderist state, giving people with same SEX atraction a choice between poisoning and mutilation, (aka gender reassignment) or death. Genderist 'liberals' have been known to criticise western women for choosing the traditional (since the 1960s) short hair, flat shoes and trousers presentation style many of us grew up with.
Basically they hate woman - even female genderists hate women.
A person who approves of gender stereotypes is, by definition, a genderist, whether they are homophobic, or gay, or with gay friends, trans, friendly with individual trans people, or hostile to trans people. If they believe in gender, they are genderist.
Gender critical people do not believe in gender, or in the concept of 'a sense of gender'. Gender critical people believe in sex as a reality, and also believe that sex is not a thing that should define a person's clothes, habits, character or way of life.