en_tn/gal/04/07.md

you are no longer a slave, but a son

Paul uses the word for male child here because the subject is inheritance. In his culture and that of his readers, inheritance passed most commonly, but not always, to male children. He was neither specifying nor excluding female children here.

you are no longer a slave ... you are also an heir

Paul is addressing his readers as though they are one person, so "you" here is singular. (See: rc://en/ta/man/jit/figs-you)

heir

The people to whom God has made promises are spoken of as if they were to inherit property and wealth from a family member. (See: rc://en/ta/man/jit/figs-metaphor)