Gender is one of those big themes that is a little difficult to nail down, in part because it often informs a text without being explicitly addressed. Of course, there are instances in Sula in which we do get direct commentary on gender and gender roles, particularly as they relate to the limits placed on women. But more often than not, we have to dig a little deeper to understand the gender dynamics. And it's not just men who are invested in upholding certain gender roles while dismantling others. Some women are just as invested in maintaining these societal roles, no matter how restrictive they seem.
Gender is the central issue in Sula. It dictates how characters relate to each other and how we as readers relate to them.
Issues of gender inform the story, but they don't drive it.