Even young children know about the differences between girls and boys. Teach your baby the sign for the word girl in baby sign language with the following description and video.
Form the hand into a fist with the thumb sticking up. Rub the thumb on the face from the cheek to the chin.
Beyer, Monica. 2006. Baby talk: A guide to using basic sign language to communicate with your baby. New York: Jeremy P. Tarcher/Penguin.