Final answer:
The literary device used in the phrase is irony.
Step-by-step explanation:
The literary device used in the phrase "Gatsby, who represented everything for which I have an unaffected scorn" is irony. Irony is when there is a contrast between what is expected or intended and what actually happens or is said. In this case, the speaker expresses a strong disdain for Gatsby, but the use of the word 'represented' implies that Gatsby holds some significance or importance in the speaker's life.