Simply put the color green symbolizes hope. Green is often used to portray new life. When babies are born or spring emerges, there is always great hope for what it to come of the new time that has just started.
For Gatsby, the green light that stood at the edge of Daisy's dock symbolized the hope of a new time with her. He had spent years preparing himself for this new time, and he would spent hours looking at the light believing in not just the potential of a future with her, but the absolute certainty that he would get her.
Green can also be a color of promise or renewal. Each of these ideas further fit Gatsby's reason for being drawn into that light.
In chapter 5, the attainment of hope for Gatsby begins as it seems he and Daisy have agreed to begin some sort of relationship together.