The symbol represented Daisy, so close yet so far from Gatsby, watching her from a far knowing she's there but she not knowing him there. Once finally having her in his arms he saw the light as a normal light because now she's not so far away but right where he wants her.
"You always had a green light that burns all night at the end of your dock." He seemed absorbed in what he had just said, possibly I had occurred to him that the colossal significance of that light had no banished forever. Compared to the great distance that had separated him from Daisy it had seemed very new to her, almost touching her. It had seemed as close as a star to the moon. Now it was again a greenlight on a dock." (Fitzgerald, 98)