Yes that does sound very 3rd wavey, choosy choosy choice 'feminism'.
In reality we're not that free to accept or reject 'gender' because it is imposed on us from birth. It's not just how we express ourselves, it's what is done to us and the ways in which we are punished for failure to comply.
These are some excerpts from Julia Long's talk in 2017 where I think she really nails what gender is:
It kind of annoys me when people talk about 'gender is a social construct' because I think that's very wishy-washy - it's not just about a social construct that affects men and affects women. It's about relations of power between women and men, obviously. We do not live in an equal world of power relations between women and men. So gender is about cementing and embedding and maintaining and reproducing the particular power relations between women and men.
... when we think about masculinity what we're thinking about are all the kind of behaviours, so-called attributes, maybe styles of clothing, certainly status, position, a sense of potential and possibility in terms of your power and what you can achieve. But what we're talking about is a whole collection of things which together form ritualised male domination. That is what masculinity is, it's not an individual gender identity trait, it is a set of different kinds of practices and codified behaviours and systems that ritualise male domination. By contrast, femininity is ritualised female subordination ...
So, as Lierre Keith says, it is a real mistake, and this is such a kind of foundational point of the whole transactivist narrative, that gender is a binary, or gender is a spectrum and then you can be non-binary or anywhere along the spectrum. Gender is certainly not a binary it is a hierarchy because it's cementing those power relations with men on top and women on the bottom and it is the means by which those power relations are naturalised, institutionalised and eroticised. So it seems to be natural, through things like religious or scientific texts, it's institutionalised in all these institutions that we can think about, and it's eroticised because it is key to heterosexuality as well, that this relationship with subordination and domination is seen to be somehow sort of intrinsically linked to sexual desire and - yes, it's seen as desireable in that sense.
So as I say then, what gender is, is a political system of male domination and female subordination. That's what it is. I don't think we should use the term gender, I think it would be good to just abandon it and when we're talking about patriarchy, which is what we are talking about, we need to talk about male domination and female subordination. Gender is just a very unhelpful word that serves to obscure the reality of what's going on and to set up other kinds of really meaningless categories. So it's a means of institutionalising, naturalising and eroticising male supremacy and what I'm arguing here is that what gender is not is some kind of innate property of the individual, as in terms like gender identity or gender expression.