Nick is the narrator in The Great Gatsby. If it wasn't for Nick, we wouldn't have the background information on Gatsby we get in Chapter 6, and this is of vital importance because it shows his progression from James Gatz to Jay Gatsby.
Nick is also a very reliable narrator. Despite the fact that he is a character in the novel, he is able to maintain a relatively objective perspective on the events of the novel. In fact, he prides himself (without seeming too proud or pretentious) on this fact. At the beginning of the novel, he notes this quality, one that indirectly comes from the lessons of his father:
In consequence, I’m inclined to reserve all judgments, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores.
Nick is therefore not judgmental in the pejorative sense. Nick does make judgments but they are relatively unbiased. He examines particular things such as the nuances of Gatsby's and Daisy's gestures. He recognizes symbolic features in some of the other characters' (namely Gatsby's) lives. For example, Nick notices that when Gatsby and Daisy are reunited, the beckoning green light at the end of her dock has lost, for Gatsby, its symbolic significance.
Compared to the great distance that had separated him from Daisy it had seemed very near to her, almost touching her. It had seemed as close as a star to the moon. Now it was again a green light on a dock.
Throughout the novel, green becomes symbolic of Daisy, the American Dream, and money. Nick subtly points these symbols out and in this way, he acts like the reader's guide as he deciphers the other characters' behaviors, revealing their motivations and desires.