Alberto: I'd advise you to look at two themes:
1. Although the GF has the Mafia as a defining milieu, it's not a book about the Mafia--it's a book about a family working within the Mafia. Think of "Gone With the Wind": The U.S. Civil War is the defining milieu, but GWTW is not a Civil War novel, it's about a family whose existence is defined by the Civil War.
2. The Mafia closely follows the rough tactics of American business and industry, particularly in the so-called "Robber Baron" era. Notice how many times characters in the GF refer to "business," as in "It's not personal, Sonny--it's business"; or, "Tell Mike it was business--I always liked him." Michael tells Kay that his father is like any other businessman who is responsible for others.