The idea that gender is entirely socially constructed is easily the greatest misconception about gender that gets repeated time and again -- almost always by cis people, who never think too much about it because they've never had to reconsider their own gender.
Gender roles and gender stereotypes really are socially constructed, like the idea that some clothes are feminine and others are masculine, just to name one example. Gender identity, however, is not. If that was true, like the previous commenter was saying, conversion therapy for trans people would work, when it's been shown it absolutely doesn't. Gender dysphoria isn't a social construct either. Many trans people see their own lives improve considerably after taking HRT (hormone therapy) and having gender-affirming surgeries -- how can that be explained socially? Also, we know there is a genetic component to being trans as well, because of twin studies. All of which shows there really is a biological component to gender -- just not in the "gender = genitals" way that transphobes think.