Is Fitzgerald being literal or figurative here? What is he trying to say about the “valley of ashes” and the people who live there? Consider the way Fitzgerald has described the Eggs compared to the way he describes the valley they drive through to get to

I think Fitzgerald is being both, literal and figurative. Literally, the Valley of Ashes is a massive trash dump. The rotting valley represents the rotting American dream. Tom, Daisy, and Jordan, with their endless socials and superficial lives represent how material excess rots their very souls. Their lives, like the valley, are hollow. As time goes on the valley gets ranker as do the lives of many characters in the novel. Self-centered individuals litter the story with meaningless "trash", much like the valley itself. Figuratively, it is the American Dream: rotten and futile.