Nick's introduction of himself and his story suggest early on that he expects to find a world of sophistication when he moves east. Instead of finding sophisticated adults discussing big ideas, he finds a petty tyrant in Tom and a dramatic, selfish, materialistic false sophisticate in Daisy.
In the first chapter, Nick recounts the idea that he intended to become a "well-rounded man" in coming east after college. He brings books and ambitions with him and plans to study finance from books that "stood on [the] shelf in red and gold like new money from the mint, promising to unfold...shining secrets..."
Clearly, Nick has high hopes for life in the east. These hopes are specifically associated with learning and, to a lesser but still significant extent, with a sense of maturity.
Instead of finding his cousin and her husband to be a fulfillment of his expectations, Nick finds Tom reading racist books and becoming fully invested in the ideas expressed there.
He tells Nick that, based on a book Tom has read and obviously reveres, “The Rise of the Colored Empires,” civilization is “going to pieces” and that the white race will be “submerged.”
He finds Daisy in a situation where she is aware that Tom is cheating on her, yet she decides to stay with him and play the role of the pained wife, demanding attention and seemingly content with it as her due and her satisfaction.
In this way, Nick's surprise is explained. Tom is not what Nick expected. The brutality of the man and the coarseness of his ideas (and his life) make Tom something far from the sophisitcated and mature person Nick expected to encounter.