India's "hjira" is real, documented evidence that the whole concept of "gender" is nothing more than just that: a social concept that varies with different cultures.
You, being in the society of America, would see a hjira individual as either male or female, while the CULTURE/SOCIETY of India would see them as a separate, third gender. I'm not entirely sure what's so difficult to understand.
Arguing against "gender is a social construct" at this point is just being contrarian for the sake of contrarianism. There is literal proof that it is.