Nick Carraway, as narrator, opens the novel with details that indicate a memoir or autobiographical work of literature--and this is his story, but contained within his story is the story of one Jay Gatsby, whose dream of wealth and greatness goes back to his late teens/early twenties and end tragically in a swimming pool at his mansion. Nick goes to New York an idealist dreaming of some of the same things that have so appealed to Gatsby, and he is swept up into the world of Gatsby, the Buchanans, and Jordan Baker, whose beauty, haughty attitude and independence entrance him. Nick's story in some ways mirrors Gatsby's, and when Gatsby's life ends, Nick's illusions end as well. The end of Gatsby's story sets into motion the end of Nick's foray into New York's seedy world of new money and shallow, vapid people; he contemplates the ruining of the American dream as he looks out over the island of Manhattan before he goes home to the Midwest.
I'm going to answer this to see if I get it right,.
In the first chapter, Nick appears to be telling his own story, but he actually tells Gatsby's story, which is framed inside Nick's story. Nick is an unnecessary character, who could be eliminated by telling the story from another character's point of view.