I wrote an essay about hope, envy, and greed in The Great Gatsby and I don't know how to conclude it. Here is my introduction: "The Great Gatsby is...

The conclusion should be similar to the introduction but it should include some information that is included in the middle portion of your essay. Given the themes of hope, envy, and greed, your conclusion should note how one or more of the characters exhibited these traits. And since you note the symbolism of the color green, you these were applied to Gatsby's life. 

Greed seems too strong a word for the idealist Gatsby. It is easy to associate greed with Tom and someone like Wolfshiem. But Gatsby's attempts to attain money were made in order to get closer to Daisy. So, he is driven more by hope, love, and envy than greed. The green light at the end of Daisy's dock represents hope to him. It symbolizes her and their potential reunion. It is an idealistic and pure symbol. But as much as he idealizes her in this romantically pure way, he does associate her with money. In Chapter 7, he says "Her voice is full of money" and Nick adds: 

That was it. I’d never understood before. It was full of money — that was the inexhaustible charm that rose and fell in it, the jingle of it, the cymbals’ song of it. . . . high in a white palace the king’s daughter, the golden girl. . . . 

Of course, Gatsby is "green with envy" in that he is jealous that Tom is with Daisy. And the green light symbolizes hope. This notion of green as pure romance and hope might also be associate with plants, foliage, and the vibrancy of life. But green also symbolizes money and greed. Gatsby idealizes Daisy but he also thinks of her association with money. And this suggests that his dream, and the American Dream in general, is corrupted by a system based upon money and greed. So, maybe your conclusion should summarize how Fitzgerald weaves these different symbols of "green" together in this take on the corrupted or conflicted American Dream.