You can read more about this in our post all about the green light.
is a tragic love story on the surface, but it's most commonly understood as a pessimistic critique of the American Dream.
In the novel, Jay Gatsby overcomes his poor past to gain an incredible amount of money and a limited amount of social cache in 1920s NYC, only to be rejected by the "old money" crowd.
However, in Chapters 7 and 8, everything comes crashing down: Daisy refuses to leave Tom, Myrtle is killed, and George breaks down and kills Gatsby and then himself, leaving all of the "strivers" dead and the old money crowd safe.
Furthermore, we learn in those last chapters that Gatsby didn't even achieve all his wealth through hard work, like the American Dream would stipulate—instead, he earned his money through crime.
You can read a detailed analysis of these last lines in our summary of the novel's ending.
But I didn't call to him for he gave a sudden intimation that he was content to be alone--he stretched out his arms toward the dark water in a curious way, and far as I was from him I could have sworn he was trembling.
He then gets killed after being tangled up with them.
Through Gatsby's life, as well as that of the Wilsons', Fitzgerald critiques the idea that America is a meritocracy where anyone can rise to the top with enough hard work.
The 1920s in particular was a pretty tumultuous time due to increased immigration (and the accompanying xenophobia), changing women's roles (spurred by the right to vote, which was won in 1919), and extraordinary income inequality.
The country was also in the midst of an economic boom, which fueled the belief that anyone could "strike it rich" on Wall Street.