In any sort of medical context yes, gender is meaningless.

In social or grammatical contexts it's a concept, similar to race, that effects how people see each other. Of course unlike race with has practically no basis in scientific reality and is a purely social construct, gender is unequivocally related to the concept of biological sex which is binary and doesn't vary across cultures.

Different cultures may have different practices concerning the relationship between the sexes which does allow for some forms of relationships that don't purely fall along the nominal male and female reproductive divide, eunuchs being a prime example. Though eunuchs are still unequivocally male. And their existence as a social roll doesn't mean they are viewed as a third gender anymore than unmarriageable women or nuns are viewed as a third gender.