Beginning with the natural world around us, green has always been used as a color to symbolize life - plants are green when they are alive and growing, many animals depend on green leaves for food, we eat plants that have lived and grown and matured.
For Gatsby, the particular green light, "minute and far away, that might have been the end of a dock," symbolized not only life but his hope and dream for the future. It is implied, although never specifically stated, that Gatsby's mansion was purchased because he was aware that he could see the dock of the mansion belonging to Daisy, the subject of his dreams and the light of his life. "He was content to be alone" while staring at that green light because, in his mind, he wasn't alone. Dreams of Daisy were filling every fiber of his being as he looked across that bay.