Nick Carraway is our first-person narrator, but he's not the center of the story—and that makes him a peripheral narrator, someone who's always on the outside looking in. He tells us at the beginning of the first chapter that "I'm inclined to reserve all judgements, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores" (1.3). Translate? People like to tell Nick their stories. And boy do we get stories: Gatsby's story, of course, but also Tom's story, Jordan's story, Daisy's story, and even the story of the Wilsons.
Ultimately, Nick's major character trait – reserving judgment – allows him to be almost an "invisible" narrator, similar to a traditional third-person omniscient point of view. Which leaves us with a question (or three): why choose a first-person narrator at all? Why not just a third-person and be done with it? And how "invisible" and "non-judgmental" is Nick, really?