(GC stands for “gender critical”, when you see that it usually means that you’re talking to/about terfs who are trying to rebrand their image).
For those of you who are wondering who else the original tweet is referring to, they are talking about Margaret Atwood, a very prominent feminist author (probably best known for The Handmaid’s Tale). Because she is a feminist the terfs assumed that she’d naturally side with them, and it came as a massive shock (for some reason) when she very publicly opposed and humiliated all of terfdom.
But yeah I don’t know how terfs can possibly get through Discworld books without any kind of self-awareness but apparently they do. I’ve also seen them talking about how the Wives in the Handmaid’s Tale are all trans women (solely based on the fact that they are infertile). If you know anything about the plot or even the basic premise you’ll know how ludicrous that suggestion is.
Terf rhetoric basically revolves around throwing shit at the wall and then crawling around in anything that sticks, but the underlying transphobia is always present.