In Chapter 1 of The Great Gatsby, Nick sets himself up to be operating as the novel's superego, the morality principle: "I’m inclined to reserve all judgments." He takes pride in not judging people, (lest we judge him). Really, it's a set-up and an excuse for him to be privy to gossip.
Later, in Chapter 3, he distinguishes himself from Jordan Baker, who he calls "incurably dishonest": "Dishonesty in a woman is a thing you never blame deeply." He ends the chapter thusly:
Every one suspects himself of at least one of the cardinal virtues, and this is mine: I am one of the few honest people that I have ever known.
So, Jordan--because she cheats and golf, is attracted to gossip, and lives carelessly--is the id to Nick's superego.
In fact, compared to Nick, the book is full of ids: Tom (drinker, woman beater, adulterer, racist, materialist), Gatsby (bootlegger, liar), Myrtle (adulterer), and George (jealous murderer). All women are temptresses, and all men are jealous liars.
But, Nick somehow forgives Gatsby's id-driven behavior because it is so romantic and boyish. Gatsby, therefore, reminds Nick of his earlier id stage as a boy, when he dreamed of a girl and getting-rich-quick. Nick buys into Gatsby's lies, that his parents died and left him money. It's the American (Oedipal) Dream!
In reality, Nick has been operating in the id-in-denial stage for his entire narration. He is complicit in most of the crimes in the novel, including slander. He fails to take a stand against Tom's racism, adultery, woman-beating, and scapegoating. He arranges the affair between Gatsby and Daisy (he's a pimp!), and he kisses on a notoriously dishonest woman. In the end, he denounces the Buchanans for hiding behind their money, but it's tantamount to whining by that point.