I believe, and people can correct me, that initially it was used by a group of feminists that used the term to refer to the fact they were 'critical of gender' as in 'gender stereotypes'. Those stereotypes which have been used by male people who don't feel like they are 'men', so feel that what they must be is a 'woman'. Those are like (a very crude example)
Feminist: The dishes need washing, anyone can wash those dishes.
Importantly, all these 'man' and 'woman' identities are only ever the perception of what that person believes a 'man', 'woman', 'girl' or 'boy' feel. It is only what they want those words to mean. It may be somewhat close to material reality, but it really is not close and is based on gender stereotypes. Because what is this 'feeling' ? It is feeling like they are not the sex they are, it is only logical that they can only then assume they 'feel' like the other sex. Despite it being only ever their own construct of that sex.
To use the above as an example they might say:
Extreme trans activist: I washed the dishes, I must be a woman
Currently, due to extreme trans activists efforts to discredit feminists, the term 'Gender Critical' has been expanded by those activists (again, happy to be corrected on this) to include 'everyone who disagrees that people can change sex'.
This is a problem. Because those extreme activists have used this expansion to silence and shame women / feminists by forcing an alignment on a 'universal belief'. It is proven science that people cannot change sex. Ever. It is therefore a universal belief and is held by the majority of the world's population. Including far right people, sometimes religious conservative people, who actually embrace gender stereotypes.
Therefore , this forced expansion now includes people who AREN'T critical of gender at all. They still hold the universal belief. They just have very different motivations for speaking out against law and policy changes that they feel prioritises 'gender' over 'sex'.
Those now in the 'gender critical' group but are not actually critical of gender might say:
Embracing gender: I am a woman, I will do the dishes
Either way, now many people do not accept the label 'gender critical', while still many still do. But it is like the word 'woman' and 'tolerant' and many other terms, it now includes the 'opposite' of its intention in its definition by some groups in society.