MissBarbary
If we describe "masculine clothes", that's shorthand for "masculine (as they are generally defined)".
As you say, the GC position is that there is no inherent gender – you have a sex, and then there are cultural gender norms that are traditionally associated with or imposed on the sexes. Whatever your sex, you should be able to wear and do what you like from the whole range of "gender", just like you can have any music taste, or any taste in food.
However, modern trans ideology tries to bring back those strict sex-defined gender norms, and say that (for example) if you're born a girl but like trousers, ties, engineering and short hair, and/or maybe are a lesbian, then you are ort could be "really a boy". That's based on nothing other than sexist gender norms. So while we'[re engaging with that position, it makes sense to talk about "masculine" norms - i.e. those traditionally identified as masculine - when we are pointing out that those gender norms traditionally associated with boys don't make you a boy.
I also actually think that there is nothing wrong with expressing yourself through the gender norms of your choice, even if that does mean being a "feminine" girl /woman or a "masculine" boy/man. These traditional associations do clearly exist and fashion/culture often plays with or subverts them - as you'd expect. You can recognise that without abandoning the GC position.
Since you think we don't get it, can you explain clearly why you think it does make sense to suggest a person is trans on the basis of sexist gender roles and stereotypes?