There are a number of symbols in The Great Gatsby. Here are a few.
The green light symbolizes growth, progress, and money. Gatsby stares at the green light and it comes to be like a beacon for him, a symbol of Daisy and what she represents for him: the American Dream. Because Gatsby felt he needed to obtain money in order to be suitable for Daisy, green also symbolizes money and is perhaps a foreshadowing symbol of the corruption and recklessness with which Gatsby pursues this dream.
In the next chapter, the symbolism of green is juxtaposed to the grey of the Valley of Ashes and this represents the failure of the American dream, a land of forgotten opportunities.
In Chapter 3, the books symbolize Gatsby's persona because they look like cardboard but are in fact real. Gatsby is a real person but with a constructed identity.
Daisy is (for Gatsby) an idealization, a holy grail, the American Dream. In the broader context of the novel, she represents the realty that the American dream is unobtainable, or to be obtainable, one must be willing to embark on a path of corruption.
Another symbol, first described in Chapter 2, is the billboard of Doctor T. J. Eckleburg, ironically an oculist whose picture on the billboard "stares" over the bleak landscape of the Valley of Ashes. "They look out of no face, but, instead, from a pair of enormous yellow spectacles which pass over a nonexistent nose." America has often been described as the promised land, a place of opportunity. Doctor Eckleburg's eyes without a face staring out from the billboard symbolize a God who has essentially forsaken the land and people, leaving a lifeless poster of himself in his place.
The green light (and Daisy) are symbols with drive Gatsby to continue his pursuit of his dream. The Valley of Ashes represents the dreams which failed. Some symbols drive the plot, but the landscapes (green and gray) and the billboard illustrate the world Gatsby lives in. By illustrating the corrupt world of East Egg and the desolation of the Valley of Ashes, Fitzgerald sets Gatsby on stage for an empty/corrupt victory or a bleak defeat.