Nick Carraway is the narrator, and Gatsby's story is told through Nick's eyes. At the same time, readers can follow Nick's character changes from the beginning of the story, when he is a young man coming east to seek freedom, to the end of the story, when, mature and jaded, Nick decides to return to the Midwest and settle down. We can say that Nick is a dynamic character, because he changes through the events of the story, and we can follow his character arc, or the track of those changes.
In Chapter One, Nick discusses his decision leave his home in the Midwest and go east to learn the bond business. Nick was happy in his small-town home until he went away to fight in the First World War. When he came back, he felt restless.
"Instead of being the warm centre of the world, the Middle West now seemed like the ragged edge of the universe — so I decided to go East and learn the bond business."
We can infer that Nick was a naïve young man; he couldn't have seen much during the war, since his war experience made him restless rather than giving him PTSD.
Through the next few chapters, Nick remains curious about the parties going on next door and their host. There are a lot of rumors about Gatsby, and Nick doesn't seem to have an opinion about them. When Gatsby offers Nick an opportunity to make a bit of extra money working for him, Nick respectfully declines:
"I realize now that under different circumstances that conversation might have been one of the crises of my life. But, because the offer was obviously and tactlessly for a service to be rendered, I had no choice except to cut him off there."
Nick is an honest guy in Chapter Five, and although he doesn't seem to mind his neighbor being involved in perhaps illegal business, he doesn’t want to get involved himself. This quote shows that, looking back, he knows this was a bigger deal than he understood it to be at the time. This shows that in Chapter Five, Nick is still naïve.
At the end of the novel, Nick realizes that the East is just a big circus, and that the only person with any integrity is his lovelorn criminal friend, Gatsby. In Chapter Eight, he says to Gatsby, "They’re a rotten crowd...You’re worth the whole damn bunch put together." This shows that Nick sees everyone in the east as rotten, and sees Gatsby as good.
At the end of the novel, Nick's final words are, "So we beat on, boats against the current, borne back ceaselessly into the past," showing that after everything that happened, Nick has become cynical. He started out the novel chasing a dream, the dream to escape the dreary life of a Midwest town and seek new experiences, and at the end of the novel he feels that dreams are nothing but reflections of feelings brought on by past experiences that we can never relive, no matter how hard we strive. Perhaps Nick's brief war experience gave him a sense of adventure that he wanted to relive with his foray to the east, but his experience with Gatsby showed him that New York City is a criminal playground and that he belonged back home with his family who loved him.