I don't think it's a mafia novel at heart. With Stracci and Cuneo placed away from New York City and minimized to almost nothing, what you have are three warring kingdoms set against a mafia backdrop.
I agree that GF Trilogy and the novel are not about the Mafia
per se. I put it a bit differently: GF is a saga about a family, with the Mafia as a defining milieu. To make an analogy: "Gone With the Wind" isn't a Civil War movie and novel
per se, it's a saga about a family with the Civil War as a defining milieu.