Nick is set up to be, as the narrator, the moral center of the novel. He has been raised in the midwest, a place that is set aside in the novel as being somehow separate from the setting for the novel, a place that maintains its morals just as Nick has. He notes that he tries to maintain a sense of detachment, and it is clear in the way that he describes the people in the story that he does not judge people quickly or definitively and the reader is allowed to form their own judgments. He is also honest, insofar as the reader can tell, as he relates the events and also opens up his own personality to the reader that lends further credence to his place as a moral actor.
I don't think Nick is non-judgemental. Just look at the descriptions of Tom "a cruel body" and "broken up against Tom's hard malice" or how his descriptions change from "gleaming white" (i.e. positive) imagery at the start of the novel to the "harsh and dry" (i.e. negative) descriptions at the end of the novel. Clearly he has judged Tom and the East as harsh and unattractive, so he returns West. That is a judgement. A characteristic embeded in our very nature. As for honesty, we simply cannot know with Nick because everything is told via his perspective in the novel.