In terms of theme, let's take a look at the lives of the main characters. Both Gatsby and Hester were alienated from society and not valued for their true worth as individuals. Society held grudges against each of them. Each endured an identity crisis that led to their downfall.
For Gatsby, this is evidenced by his endless pursuit to earn Daisy's love. He cared nothing for all the other people who traveled in and through his life. His funeral is indicative of this singular purpose he had in life. He never became a whole person because he wouldn't just value himself for who he was. Although people liked him, they never really got to know him. They took advantage of him for his material generosity.
For Hester, she refused to confess to her lover's identity. This caused her to experience isolation from society. She too, had no real relationships, except for the occasional meeting with her daughter's father. She seems to want to be changed in the eyes of her society, and it does slowly happen as readers are informed that the A which once stood for Adultery now stands for Able.
Both characters demonstrate great determination. When looking for themes, main characters are great illustrators for the reading audience. Their lives and actions play out the themes.
Rhetorical strategies refer to the author's method or style used in order to convince audiences of his messages or purposes (which are most often illustrated through theme).
Hawthorne uses vividly descriptive diction and complicated sentence structures. This helps build the complexity of the situation and relationships between characters. Both fill their pages with symbols: Daisy's green light, Hester's letter A, the destructive cars, the eyes of Dr. T. J. Eckleburg, and Pearl's demonic nature. Fitzgerald's similes and metaphors help readers relate to that which he describes. Fitzgerald also uses weather and colors to indicate more about a subject than is blatantly told in the text.
Both authors use their storylines to criticize an era in American history in which the societal norms have been recorded as flawed. The shame cast by the Puritans and the material focus of the Roaring 20s are both evidences that too much of something is probably detrimental.