What's so regressive about people not being constrained by the gender identities they were assigned at birth?
No one is assigned a gender identity at birth. People have their sex observed at birth and then, to a greater or lesser degree depending on their society, family etc, are subjected to gendered expectations and treatment. GC feminists say that people should not be constrained by these gender stereotypes and expectations. So, for example, that means it's fine for a boy to play with dolls, love flowers and maybe grow up to be a fashion designer, just as fine as it is for him to be a footballer or a mechanic or a nurse or anything else. For girls, that means girls should not be required to be "girly", keep quiet and serve others' needs, wear heels, wear make up, whatever and should be scientists, pilots, engineers etc if they want to.
This is all about NOT HAVING TO CONFORM T REGRESSIVE GENDER STEREOTYPES if you hadn't noticed!
The only difference between this and transgenderism is that for some strange reason, the genderist argument is that your gender somehow is your sex! So if you're a girl who looks/acts "like a boy" then you might not be a girl and should be encouraged to be "trans" and get hormones and surgery to make you into a facsimile of a boy, while being led to believe that you are actually "a boy".
THAT is regressive gender stereotyping. Because it says if you don't fit a feminine stereotype, you can't be a girl. So you can't break out of the stereotype at all. You have to have your appearance/interests/personality labelled "boy" and then have your body damaged to try to make it match the male sex because these things are "for boys".
Can't you see that that's harmful and restrictive?