The Green Light is a light at the end of Daisy and Tom's dock. This light represents dreams and wants.
The Eyes of Dr. T. J. Eckleburg
"he stretched out his arm toward the dark water in a curious way, and, far as I wad from him, I could have sworn he was trembling. Involuntarily I glanced seaward - and distinguished nothing except a single green light, minutes and far way, that might have been the end of a dock." Page 20 & 21.
Example 1 of The Eyes of Dr. T. J. Eckleburg
"Gatsby believed in the green light, the orgastic future that year by year recedes before us." Page 180.
Example 2 of The Eyes of Dr. T. J. Eckleburg
The Eyes signify God and him watching all the characters in the novel.
"Evidently some wild wag of an oculist set them there to fatten his practice in the borough of Queens, and then sank down himself into eternal blindness, or forgot them and moved away.” - Page 23/24 "
"But above the gray land and the spasms of bleak dust which drift endlessly over it, you perceive, after a moment, the eyes of Doctor T. J. Eckleburg." Page 23