Who was the narrator of The Great Gatsby?

The narrator is a homodiegetic narrator.

Nick is the narrator, but he is also part of the story, which is what a homodiegetic narrator is. Nick acts as a witness to the events, he observes and record the events without being involved personally. It can be said that Nick's narration is unreliable as he does not narrate the events as they happen, but changes the order of events (aporia). In this way he is able to incorporate proleptic (flash-forward) events. Nick also contradicts himself. He initially says that he 'reserves all judgement' but goes on to judge everyone and everything in the novel. For example, Myrtle, whom he consistently calls 'Mrs Wilson' to express his disapproval of her affair with Tom.

I hope that helped.