I think @Quz is fairly new to these boards and I hope they stay around, because I think they have more in common than not in common with GC feminists, they just haven't been around the discussion here enough to understand the terms and the stakes involved. I think if they went to see the break it down for me thread, they'd agree with more of it than they disagreed with.
A few years ago, sex and gender were words that could be used interchangeably with no real problems. Since the advert and growth of gender studies and the exponential rise of young people believing that they are trans and being told that it's no big deal if they want to identify other than their bio sex, things are significantly different.
Commonly agreed definitions on mumsnet:
Sex - biological, binary. Sometimes a disorder of normal binary sex development happens and sex is harder to identify, though this is rare and even more vanishingly rare that a person with a DSD cent be identified as either male or female sex after some investigation. There is no third sex, and sex isn't a spectrum.
Gender = gender identity. A social construct or an identity one adopts. Most GC feminists aren't fans of the concept of gender identity for multiple reasons, the main ones of which are: 1) it's part to play in the erosion of female sex spaces and rights, and 2) it's reliance on regressive stereotypes about sex. Women have long hair and wear make-up, men wear suits and like fast cars, and so on. We are critical of the concept that an innate gender identity exists and that it is has taken precedence over sex in society in recent years. Whereas previously women could tell a man entering their changing room or toilets to leave, in modern times that man can claim to identify as a woman, abuse the actual woman in the room and get her chucked out, fired and blacklisted, because his feelings of gender identity apparently trump her sex-based rights.
The problems with gender identity and preferred pronouns taking precedence over sex fact and sex based pronouns are widespread. If the term woman as a sex descriptor is eroded in widespread society into a meaning more akin to gender identity, it becomes accessible to anybody who wants it, and the consequences disadvantage biological women.
A world in which a biological male can be a woman and go by female pronouns, is a world in which biological women's rights are surrendered. Biological men enter and win in women's categories of sports, male rapists get housed in women's prisons, and awards and grants intended for women go to men.
Meanwhile teens are sold "change your gender identity!" as a harmless solution to normal teen awkeardness and discomfort. This has led to experimental pharmacology on teens (puberty blockers), cross-sex hormones with very little acknowledgement that a previously healthy sexed body may not cope very well with that, and irreversible surgery on sex organs because sex and gender identity is still very much conflated. Meanwhile the kids end up no happier, but with a bunch of additional problems.