The terminology of the debate gets confused a bit because while indeed the expectations of people’s behavior, dress, etc. based on their sex is indeed a social construct and has been probably since the beginning of humanity, that wasn’t what “gender” meant until very recently. As far as I can tell, “gender” and “sex” were completely synonymous until 50 years ago, even in niche academic communities, and until maybe 10 years ago among the general public.