"The valley of ashes". The place where Myrtle and George live, a small, desolate town on the road into the city of New York, is ash colored and ugly, so it seems not only to be sad and desolate, but burnt- and used-up in the hot summer sun. While East and West Egg have beautiful (probably irrigated) gardens, there is nothing like that in the sad little town where Wilson's garage is.
There is also a definite undertone of this town being a dumping-ground or "ash heap" of life in between the places of the living; New York, and East and West Egg. An ash heap is synonymous with a garbage dump -- and the unimportant, poor people who do not have estates further east down the island, or jobs in the city, are left to rot in the valley of ashes. Also, there is a suggestion of the cemetery about the name; Myrtle dies here, run over by Daisy in Gatsby's car. Myrtle is burnt up by Tom's use of her, and tossed away when she was no longer useful or became more trouble than she was worth. Her body was left by the road in the valley of ashes, as an animal fit only for the "ash heap". This phrase provides a rich mine of commentary about the people and places in the valley and around it.