So, from what I understand, the Green Light was supposed to be imaginary - correct? Neither Tom nor Daisy knew what he was referring to when he spoke of the Green Light - but what was its significance? Is it supposed to signify his ever burning desire for Daisy? Or something else?
For Gatsby, the green light represented his unobtainable desire to have Daisy. While the green light was really there, it serves a thematic purpose.
Gatsby put all his hopes and dreams to have a "respectable" life with Daisy into the image of the light. Plus, the fact the light was so far away...something he could see but not quite reach...also represented everything he wanted but couldn't quite have.
Whereas for Daisy and Tom, it *was* just a light. They were already what wealthy and established...so it didn't have any significance for them. There was no need to subconsciously apply hopes and dreams to it because they were 'there.
In "Vertigo" it's associated with a ghost (Madeleine) based on a theatre tradition and maybe Fitzgerald similarly employed it as Gatsby was essentially in love with a memory that haunted him.
You guys are deep. The green light is at the end of Daisy and Tom's pier. I think Gatsby decided to live where he did so that he would have that light as a connection between himself and Daisy. Other than that I agree with everyone.