The notion of gender as a social contruct has it's roots in radical feminist theory, which takes biological sex and gender for separate things. In easy terms, in the context of radical feminist theory, sex is descriptive and gender is prescriptive, with the second being built around the first.

In this context, the historical notion that women must (a limiting prescription) be stay at home mothers and not have much of a say in political matters exists because of the fact that they are the ones that can have babies, a valuable resource that every society developed ways to control via the introduction of different social norms.

In the TRA/Queer Theory interpretation of this principle though, gender is personal form of expression with no material basis that exists only by language. In their perspective, in the moment someone names a gender, that makes the gender in question exist. If I go around saying I'm stargender or some shit like that, this brings stargender into existence.

Once you go around and actually take the time to read the philosophical basis that fundaments the "gender is a social construct" argument it can guide you to a bunch of contrasting, and totally different thinkers and movements.