Well, we had to get him in here somewhere.
Nick acts as a guide for a few different people, but most of all Gatsby. He enables Gatsby's affair with Daisy, advises Gatsby that he "can't repeat the past" (8), and then tells Gatsby that he ought to leave town after the Myrtle's death. But the man just doesn't listen.
The interesting part comes at the end of the novel, when Nick says that we are all "borne back ceaselessly into the past." Because this stands in contrast to what Nick told Gatsby earlier, and because it in fact mirrors Gatsby's prior sentiments, you could argue that Gatsby in return acts as a guide for Nick. We definitely didn't see that one coming.
Dan Cody serves as Gatsby's friend and mentor. He not only gives Gatsby a taste of "elite" life, but also gives him the only education he receives – a by-the-seat-of-your-pants education that gives Gatsby the skills and experience to head off in pursuit of wealth and class.
Fun fact about Dan Cody? His name is a mash-up of Daniel Boone and Buffalo Bill Cody, two heroes of the Wild West—of the American Dream that Gatsby dies pursuing. We're wondering if Gatsby wouldn't have been better off without them.