Gatsby represents the ideal American Dream to Nick for good reasons as well as bad, which gives us something to think about as readers... although ambition is admirable, do we ever want to pursue something so seriously that it ends up hurting people?
Gatsby was successful, you could call him popular because of his parties, and Nick noted that. Nick wanted to spend time with Gatsby and hoped to be his friend. The phrase Gatsby donned on him, "old sport", made Nick feel liked.
On the other hand, Nick saw through Gatsby. Nick goes to great lengths at the beginning of chapter 1 and the end of chapter 3 to tell us how honest he is, which should make us believe his narration over anything else said.
Nick's narration notes Gatsby's nervous nature regularly: he chokes over words, he "was never quite still". Gatsby was always restless. Nick knew when Gatsby lied.
Gatsby represents the lengths people go to in order to achieve their ambitions, often people do this and succeed, even in a deceptive way. Nick defines this throughout the book.
In The Great Gatsby, Gatsby represents several things to Nick, and Nick's feelings and thoughts about Gatsby are complex.
Nick says at one point that Gatsby is everything he hates. Financially, Gatsby is the corrupted American dream. His partner fixed the World Series--messed with America's past time.
Romantically, Gatsby is naive and foolish. He's spent five years constructing his persona and life in such a way as to lure Daisy back to him, hoping to recapture a relationship that never really was.
Yet, Gatsby is also an attractive character to Nick. Gatsby's love was "pure" and all-consuming. Gatsby loves like everybody should love. And Daisy would have been better off with Gatsby.
By the end of the novel, Nick makes Gatsby's funeral arrangements, hunts down people to try to get them to attend the funeral, takes care of Gatsby's father when he arrives at the house, and, significantly, tells Gatsby he is worth far more than all of "them," worth far more than everybody else in the novel. Gatsby was special.