T.J. Eckleberg's eyes on his billboard show that someone if always watching to see you fall.
"God knows what you have been doing. You may fool me but you cant fool God." (157)This shows the billboard as a sort of God that is always watching how you act when no one else is around.
The light outside of Daisy and Tom's hows shows the greed and envy throughout the book. "..he stretched out his arms toward the dark water in a curious way, and far as I was from him I could have sworn he was trembling... and distinguished nothing except a single green light... "
This light represents the theme "you can't have your cake and eat it too."