There has been a lot of ink spilled on the symbolism of the green light. Here is some context. From Gatsby's house, he is able to see a green light on Daisy's dock. It is close but far enough away, within his grasp but not yet at hand. The novel ends with these words:
Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that’s no matter—tomorrow we will run faster, stretch out our arms farther. . . . And then one fine morning— So we beat on, boats against the current, borne back ceaselessly into the past.
For Gatsby the green light expressed his longings, desires, and dreams. In particular, he hoped to have Daisy's love. However, though he sought it, he did not make real progress, as if the current kept him in place. The point is how elusive our dreams are. The more we strive for them, the further they are. A good word to describe this is "tantalizing."
What makes matters worse is that the green light gives permission to "go." However, as we go, we are kept at bay. In the end, we get nowhere. How tragic.