There are many primary law sources (namely statutes, cases, and regulations) that deal with gender. For a nice list of some of the biggest statutes, cases, and regulations, see the following research guide from Florida State University: 

Statutes: https://guides.law.fsu.edu/Gender_and_Law/Statutes

Cases: https://guides.law.fsu.edu/Gender_and_Law/cases

Regulations: https://guides.law.fsu.edu/Gender_and_Law/Regulations