Quite a few things, actually. First is in Fitzgerald's use of color throughout the novel. This gray would represent a "dead" area, without the life and vigor that other colors of the book bring about, like green and yellow. In that sense, this is where Myrtle dies, Gatsby's dream dies, Nick's hope for something good dies, etc. etc.
This place is also indicative of Fitzgerald's (and subsequently Nick's) view of life. It is his belief that man destroys what is good. This setting of the book shows a once lively place that has been covered in soot and ash from modern industry, a visual disgrace created by man. Tying it back to the plot, this is again the place where all of our character's dreams come to a crashing halt.