The first few paragraphs of The Great Gatsby are intro paragraphs. They are dedicated to introducing the whole story. Most importantly, in these paragraphs, Fitzgerald tells us that nick is a reliable narrator. He also gives us the details about Nick. Info about his family and his background in general is shown in the paragraphs.
This s important intro to nicks life and his relation with the story as he will be doing most of the narration. The story moves on after that but without these intro paragraphs we would have been unable to develop a link between nick and the story, which is crucial.