"Involuntarily I glanced seaward-- and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock." (pg. 21)
To Gatsby, this represents his future with Daisy, one that he wants to pursue, but never reaches.
The Green Light: This physically is a light coming from the East Egg that is visible from Gatsby's house.
This symbolizes that there is always someone watching, whether on Earth, or not... meaning that everything you do is seen in some way, shape, or form, and it will always have effects.
The Eyes of T. J. Eckleburg:
This physically is a billboard made to advertise the optometrist, T. J. Eckleburg's services.
"The eyes of T. J. Eckleburg are blue and gigantic--their retinas are one yard high. They look out of no face, but, instead, from a pair of enormous yellow spectacles which pass over a non-existent nose." (pg. 23)