GF was based on the novel. Moe Green appeared in the novel, but Roth didn't. The novel ended in 1955, and so did the film. At the time GF was made, no sequel was envisioned (Paramount was fearful it'd be a flop). But after its smashing success, they decided to make II, and to flesh out the story, Roth (based on real-life Meyer Lansky) was added, as was much else.