As a review, symbolism is objects, characters, and colors that are used in literature to represent abstract ideas or concepts.
F. Scott Fitzgerald loved using symbolism to convey his feelings on human nature and the era in which he lived.
Some of the major symbols and the meanings (that I teach to my students through my research) are below:
Daisy's green light that Gatsby could see from his house represents Gatsby's hopes and dreams specifically, and since Gatsby represents all Americans, it also represents "The American Dream" of love, wealth and happiness for all of us.
The Valley of Ashes (the desolate ash-dumping piece of land) represents the moral decay of 20's and the plight of the poor.
The billboard with the eyes of Dr. T.J. Eckleberg represents the eyes of God looking down on the moral morass which was America in the Jazz Age...and judging it badly.
The unopened, unread books represent the shallowness of the rich who look good from the outside, but have no content within themselves.
The owl-eyed man could be a prophetic reference. Someone who is wise (the owl) and sees more clearly (glasses) than those around him what is happening to the society.
The colors yellow and gold are used frequently, probably to represent wealth and money of this society. White is used for Daisy often..presumably to represent her femininity and innocence.