How do Gatsby and Nick force readers to evaluate their own perceptions and beliefs?
We can approach this question by looking at how Nick and Gatsby are used in The Great Gatsby to examine the complexities of ambition and also how they are used to explore the nature of (false) presumptions/assumptions.
Over the course of the novel, Nick holds a variety of opinions on Gatsby. Before meeting him, Nick's view of Gatsby is based on rumors. As Nick gets to know Gatsby, he goes back and forth in his opinion as to whether or not Gatsby is a fraud or a good person and ultimately comes to believe in Gatsby in a way that goes beyond all of Jay Gatsby's lies and surface deceit.
Nick learns to believe in Gatsby in a way that we could call profound. The lies and the posturing matter less than his devotion to Daisy, his willingness to sacrifice for her and his dedication to a dream. This change in perspective only occurs because Nick is able to move past his initial assumptions of Gatsby's character.
Consider that Nick admits of his feelings that he once held that Gatsby "represented everything for which I have unaffected scorn" but in the end it was Gatsby's "extraordinary gift for hope" that most strongly characterized the handsome, striving dreamer, Jay Gatsby. Perhaps there is a lesson in the degree to which Nick is able to change his mind about Gatsby.
Also, many of us hold views that are based on imperfect information, rumors, generalizations or hasty conclusions. The novel asks us that, if Nick can come to discover that the truth of a person is deeper than that person's stories (or lies), perhaps we might re-examine our own presumptions and try to look at what really matters.
Additionally, we might reflect on our own ambitions with the help of Nick and Gatsby. Do we choose our goals honestly, based on a real sense of who we are and what our interests are? Or do we sometimes choose to stake a claim to ambitions that are inspired by our culture and therefore might be seen as romantic aspirations not unlike those of Gatsby?
After all, Gatsby exemplifies the idea that some dreams, once achieved, seem to be false and are "worn" like clothes that can never quite define the man or the person beneath them. Nick also went east with a half-baked notion of becoming a bondsman and an intellectual. He becomes neither because he learns that some ambitions are less honest than others (and some posturing can be dangerous).
Of course, we all want success and success is inevitably defined at least in part by the culture we live in. Yet a question arises as to what extent the glamour of success (associated often with celebrity) actually matches up with our own human substance, our emotional and psychological limits and needs. And, also, how much of our dreaming is a pure product of material striving?
"Fitzgerald's book mirrors the headiness, ambition, despair, and disillusionment of America in the 1920s: its ideals lost behind the trappings of class and material success" (eNotes).
Are your ambitions and your values truly your own or do they have a source outside of your self? Do we look for achievement (which always requires work) or do we seek instead a success that amounts to status (which is always a sense of how others see us)? These are questions that Nick and Gatsby might lead us to ask.