Butler’s claim is not that gender is a performance; it’s that gender is performative, which is a very different claim. This is a common misunderstanding, though — so common, in fact, that Butler wrote a book called Bodies That Matter to address the ways Gender Trouble (the original source from which performativity was first described). I have a piece on here called On Acting Like a Dick and Phallic Embodiment that does a somewhat more thorough job of outlining how materiality and performativity are intertwined in case you’re interested in learning more about this.