Sadly, the essay prompt is about the american dream, so that's a no go
I had to write this exact same essay on the Great Gatsby about two years ago.
You're lucky because most teachers that choose that topic allow a lot of freedom in your interpretation on how to talk about the American dream.
My theme was: it's all an illusion.
Everyone in the Great Gatsby is living under an illusion, even the main character. Tom, with his mistress, is living under the illusion that he's not second fiddle in a marriage to a women who thinks he's a bore. He goes out and finds some desperate house wife who is amazed at his wealth, and treats her like shit, because when he goes home, he's got his million dollar wife he can't touch. His mistress, btw, is under the delusion that Tom will marry her one day and make her a high class woman.
Tom's wife tries to live in the illusion that she has a happy marriage to a good man, and that her life is complete. It's obviously not, the way she wistfully talks to the main character and dotes on him.
All the supporting cast at Gatsby's parties are too part of an illusion, where they party their asses off to chase away their own woes and short comings, and stumble out the door as drunken, grotesque caricatures of what people typically think of monied wealth.
And Gatsby is the greatest illusion of all, this carefully constructed persona that disguises the real man behind the money and the legend. Gatsby ain't happy, he's miserable and his money does nothing to solve that problem for him.
The whole moral of the story, to me, was that money and success are not the end of anything, they are merely illusions which we use or which fool us into not seeing people or things as they really are. Money doesn't inherently make people happier or better people, and the American Dream is in some sense totally naive because it doesn't prepare us for or solve the problems that make many people unhappy.
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Anyways, I think I did that in about 2 pages. If you really, really cannot think of anything to write about, just ramble on about the symbolism of the Eyes of Dr. T.J. Eckleberg. It's a weird off note piece of symbolism in the book, and teachers tend to accept most kinds of interpretation about what it means, since it's just so creepy and weird and unexplained. Seriously, you can almost go the ghost of Abraham Lincoln route or something and they might buy it.
And if your summing up everything on one page when they expect two, you're probably not doing the one thing every English Literature teacher grades on: providing specific, detailed examples to back up your conclusion. If you've sat in class they know you can write 1 or 2 paragraphs about the overall theme of the book and how it applies to their chosen topic....they want specific examples to show you actually read the text and were thoughtful about its passages. I always overwrote for English courses, and only once it did ever lose me a grade. Most times it guaranteed the teacher either loved the amount of effort I put out, or were too lazy to even read it all and just gave me an A.