That depends what you classify as 'gender theory'. Although familiar with Judith Butler's earlier work, I have neither the time nor inclination to read her later (and apparently increasingly eccentric) theorising in detail. As far as I can make out, she's now come around to the idea that there is indeed an innate gender identity of some kind, although I'm more than willing to be corrected on this.
The problem with the Queer Theory that people see as such a dangerous, corrosive influence is that its interpretation by lobbyists is very like the interpretation of the Law According to Stonewall. I.e., the law as Stonewall would like it to be, rather than the law that's actually on the statute books.
Queer theory is the same, especially that espoused by its so-called godfather Michel Foucault. In fact, it had a lot of very liberating and interesting possibilities, especially for women, which is why feminists writing after Foucault in the 80s set such a lot of store by him. And he consistently held with the theory of constructivism: that 'gender' is a set of socially constructed stereotypes only arbitrarily connected with sex. It's gender, not sex, that exists on a spectrum; it's those stereotypes which we internalise as external markers of identity. There are therefore a set number of gendered 'subject positions' which are wholly historically specific - the positions from which a woman might speak in 2024, for instance, don't correspond with those existing in 1924. And we have little or no control over how those stereotypes are constructed.
'Gender' (a set of social diktats and stereotypes), is arbitrary, shifting, ephemeral and changes in accordance with social structures, geographical space and historical time. If gender shifts and changes to this extent then by its very definition it cannot be essentialist: an unchanging, unified, rational inner 'essence' of the human subject or the gendered subject. In which case, the gender critical position is constructivist. It accords with a lot of Foucault's ideas that Gender Ideology has coopted as theirs by right. It doesn't hold with the idea of an innate, core, unified subjectivity, either of the humanist subject or the gendered one. Neither did Foucault.
Conversely, Gender Ideology is essentialist by its very definition. That the GI lobby has reversed this position to fit its own interests is very interesting indeed. Its claims are built on a house of cards and a flimsy one at that - in the hope that it's too complicated for others to understand (and beyond the remit of a 180-word tweet).
That's how this disingenuous movement has gained purchase, IMO.