This is an excellent question that recognises how setting is used symbolically in this wonderful novel. Of course, the Valley of Ashes is described very symbolically to represent a place barren of hope. It is no mistake that this is the home of the Wilsons, who try to get ahead in life yet seemed to be doomed to living on a low social level. Consider how the Valley of Ashes is described in Chapter Two:
This is a valley of ashes--a fantastic farm where ashes grow like wheat into ridges and hills and grotesque gardens; where ashhes take the forms of houses and chimneys and rising smoke and, finally, with a transcendent effort, of men who move dimly and already crumbling through powdery air.
The metaphor of describing this location as a "fantastic farm" which grows ashes clearly says a lot about the reality of life in this industrial zone, and we can see through the description of George Wilson, who is a thin, pale man, somehow faded, that he is a suitable citizen of this part of town.
However, let us compare this with the way that Gatsby's parties are described at the beginning of Chapter Three:
There was music from my neighbour's house through the summer nights. In his blue gardens men and girls came and went like moths among the whisperings and the champagne and stars. At high tide in the afternoon I watched his guests diving from the tower of his raft, or taking the sun on the hot sand of his beach while his two motor-boats slit the waters of the Sound, drawing aquaplanes over cataracts of foam.
Note how wealth and its decadent display are emphasised in such passages. Firstly there is a lot of colour in this passage, compared to the grey of the valley of ashes. Likewise, almost every sentence contains the trappings of wealth, whether it be with references to champagne, motor boats or other things.
Thus we can see the way that West Egg functions symbolically to represent the home of new wealth, with all of its garrulous overtness, and the Valley of Ashes represents the hopelessness of industrial areas.
I think it's also worth noting the clear juxtaposition of the four main settings in the novel: The economic and social depravation of The Valley of Ashes, the hope and opportunity represented by New York City, the ersatz and gaudy pretension of West Egg and the haughty moneyed feel of East Egg. These are all ripe for analyses and comparison.