You have to look at the novel in total, particularly the last chapters. It's important to move beyond the flappers and parties, even the two murders. The backstory that Nick (quite impossibly, in the level of detail) discovers about Jimmy Gatz, aka Jay Gatsby, is critical to feeling both the glory and the tragedy of the story.
Gatsby, it turns out, is an unmade child of the Midwest, ashamed of what he is, and schooled in the most banal kind of mail-order self reinvention, who is then elevated and corrupted by Dan Cody and his lover. These two influences are fused in the pursuit of Daisy, and much else he does. (There is a later echo of this Midwest/East motif when Nick talks about taking the train East in college, as well. We can tell it's a pretty big deal in the book's artistic intention.)
Much of American Literature is a consideration of our ability to head to the frontier, reinvent ourselves, make a shining city on a hill, be the last best hope for mankind, free ourselves of the shackles of the past, the tragic fate of birth in a particular place ... you get the picture. It is shot through our attitudes to class, politics, the immigrant experience, and much else. That is why it is such an important theme in our national art -- you don't find it in the same way in the literature of England or Japan, say. This is rather uniquely explored in The Great Gatsby.
Fitzgerald, who was a very great artist and an admirer of John Keats (think the romance of impermanence, beauty that must die to have meaning, etc), added to our discourse on self-invention a deep expression of the romantic yearning inside this dream. In addition, he noticed the way in which we love the promise of the glittering and the shiny and the powerful, but how even to dream of it, let alone to seek it, also corrupts us and destroys us. And yet, we need it and live by it.
Art notices and points at previously little-noticed things in our experience, and helps us experience life more fully -- sometimes even more wisely. Fitzgerald added deeper meaning to understanding the problematic, often tragic, dimensions of pursuing the American dream, or experience.
He did this at a time when America was becoming even more powerful, and its promises of power, fame, and adoration even more extreme. That created a whole new dimension to our understanding of our culture and ourselves.