I wrote an essay about how the symbols of the green light, the eyes of TJ Eckleburg, and the color white relate to the theme of the corrupt...

This sounds like a great essay, connecting three symbols in The Great Gatsby to the American dream.  For your conclusion, you will need to do a little review for your reader.  By that I mean that you need to remind your reader of what your main idea, your thesis, is and also remind your reader that these are the three symbols you used to support your thesis. You need to avoid using exactly the same language you used in the thesis statement. In other words, your conclusion should not repeat your thesis statement word for word. We never want to exactly repeat ourselves in our writing. A conclusion can also make some general comments on your thesis. For example, you might want to say something about the American dream having always been futile and corrupt or that it continues to be corrupt in today's world. Or you could comment on the pain of the characters, having suffered for a corrupt illusion.  A little editorial commentary in a conclusion is often effective.  What you want to bear in mind is that a conclusion should, to at least some degree, provide some symmetry to your essay, so that you are closing as you began.