Jay Gatz was a poor nobody who didn't get the girl (Daisy) when he was younger. He remade himself into Jay Gatsby--shadily and even criminally--to "get" the girl. When he and Daisy finally reunite and have a "romance" (not really much of one, but ...), he shuts down his greatness (no more parties, no more extravagance) and tries to become his old Jay Gatz self again--but it's too late. Jay Gatz is somewhere back in Minnesota. Sure, he made a name for himself, but he burned the candle at both ends, flamed out, and ended up with less than what he started with.
I taught this book for a decade, and by the end of the book, my students didn't like or respect a SINGLE character in the book--which is fine. I don't believe Fitzgerald wanted us to like them or even to emulate them. Was Fitzgerald writing about "the American dream gone bad"? Perhaps. Gatsby's dreams glowed for a bit and winked out.
The modern parallels are astonishing. How many "celebrities" become rich, live to excess, flame out, and end up in rehab (Lindsay Lohan and many others)? How many current celebs are famous (or infamous) for simply being famous like the Kardashians or Paris Hilton? Why does society consider them "great"? What does it say about any society that elevates the rich and (in)famous to such high positions ... and why do we groundlings always cheer their inevitable falls?
My students agreed that Nick was by far the "worst" character in the novel. He said he was honest, and yet he didn't tell the whole truth. He kept secrets. His inaction led to a few deaths.
It is truly amazing that a novel this short (@ 50,000 words) can still echo and resonate today.