Sicilian Babe is right about the character insights. The novel also explains much of the Sicilian culture that gave rise to the Mafia. Puzo also did that in The Last Don and Omerta. I don't know how much of that is historically accurate.

But I think the GF novel has more of a pulp sensibility. Johnny Fontaine and Nino Valenti are (I assume) meant to resemble Frank Sinatra and Dean Martin. I found that roman à clef approach to be distracting in the book. Harold Robbins and Sidney Sheldon used to write entire novels that way. Not only did the movie leave out most of the Hollywood/Fontaine/Mancini stuff, Al Martino looked nothing like the young Sinatra.

Other than that material and the Young Vito chapter, the book and GF1 are almost the same story. One meaningful difference for me was when Michael proposes shooting Sollozzo and McCluskey. In the book, Sonny jokes with Michael and then expresses joy that Michael had finally joined the Family, so they can work together and wipe out the Family's enemies. Another difference is the last chapter, really an epilogue, which I won't give away here.