Many critics agree that the billboard advertising the occular practice of Dr. T.J. Eckleberg symbolizes the eyes of God looking down on the immoral world of the 1920's and finding it wanting. Others believe since the billboard was old and faded that it is a continued reference to the death of another American dream (the Doctor opened the practice hoping it would succeed but it obviously closed and he didn't reach his dream).
Either way, it is significant because George Wilson's dreams are going down the drain (he is losing his wife to Tom Wilson, and his garage is not doing well). He feels a moral outrage toward those around him who live much better than he does, but lie, cheat and steal to get what they want in life.
Just like Owl Eyes was able to interpret the exact amount to phoniness Gatsby displayed in his display of wealth in his home, the eyes in the billboard can interpret the duplicitous behavior in Myrtle's life. Both are desperate to be involved in the world of Tom and Daisy, but they are frauds. Wilson believes that God sees through and there is some sense of eternal justice in this modern world. Only those with "old money" seem immune to the justice distributed in this world.
The eyes on this huge billboard are used and refered to because it shows that God sees everything. This is interesting when Myrtle is involved with the car, and Nick is the only one - bar Daisy- who knows, making him all-knowing.
this is my interpretation however, and it may not be the same as yours was. let me know.