Biology does influence gender development.
For example, some differences between typical boys and girls can be explained by the effect of genes on sex chromosomes, and by the levels of sex hormones and their effect on the brain during early development (e.g., prenatal exposure to high levels of male-typical hormones). Hormonal changes carry on into puberty and adulthood.
However socialization can also play a role by enhancing or countering biological influences. For example, girls exposed to higher levels of male hormones before birth have more interest in “boy” activities, but socialization may force them to behave in a way that is more typically “girlish.”