The radical stance on the other side is "gender isn't something you are but something you do" if that makes sense.

This isn't "radical", it's literally just an attempt to uphold the status quo by reinforcing gender norms as fixed things.

What is a gay femboy doing? Is he doing the female gender? In all ways in every aspect of his life he is behaving withing feminine norms, and having sex with men. All aspects of his behaviour, of his everyday life, are indistinguishable to others as the same actions that a transwoman might perform.

Is he doing female gender? No he's not. He's just being outside of typical social norms.

Gender is not defined by behaviour.

A woman can be a bodybuilder, that dresses masc, that behaves masc, and still be a woman. She's not "being a man".

The premise of gender being defined by behaviour is just a conservative attempt to reinforce social norms that they wish to uphold because they're trying to preserve patriarchy and various other social pillars they view as essential to preserving the existing hierarchies of power.

Gender is not something you do. Gender is something you simply are, and it is incongruence with what you actually are and what you're socially perceived as that produces feelings of discomfort. Which is precisely why acceptance is absolutely crucial to positive feelings, and why suicide and depression rates in trans teens pre-op and pre-hormones that have a fully accepting social and familial network (basically their whole lives) drop to essentially near cis levels. The social factor is extremely significant.