In his description of the valley of ashes ("This is a valley of ashes — a fantastic farm where ashes grow like wheat into ridges and hills and grotesque gardens"), Fitzgerald uses literary devices to develop a musical effect in his writing. First, alliteration is used to create rhythm. Phrases such as "fantastic farm" and "grotesque gardens" use the repetition of initial sounds to capitalize on the tone that these phonetic sounds produce. Next, sibilance is used throughout the paragraph to create movement in the lines. Phrases such as "paintless days" use the repetition of the "s" sound to create this movement. Finally, Fitzgerald uses punctuation (namely commas) to create rhythm in the passage though the development of successive phrases and clauses. So Fitzgerald uses not one but several literary devices to create music in his writing.
There are lyrical qualities to the description of the valley of ashes that could reflect a type of music being developed and emerging. The last line strikes a note of dissonance, a type of bell or knell being rung with "paintless days, under sun and rain" and the idea of "brood" and "solemn" helps to bring out a minor key in tone. The manner in which the "eyes" are described could also be of musical quality, especially in its repetition in phrases such as "eyes of..." and then, "The eyes of..." It seems like the lyrics of a song in the repeating of the same words, to bring emphasis and meaning out. Finally, I would say that the entire feel of the passage is reminiscent of a the opening to Elton John's song, "Funeral for a Friend/ Love Lies Bleeding." The swelling of organ chords to open as well as the complexity of synthesizing sounds creating an entire feel of instrumental despair is brought out in my mind within this passage.
What a fascinating question. I must be honest, there is no one literary device that jumps out at me and screams "music." There is, however, a certain rythm and cadence in the description of the valley of ashes which inspire an image of musicality. If I had to put a name to it, I'd say it was personification because of the movement of objects in the piece.
First we have a road, veering toward the train tracks and shrinking away from the gray and ashy terrain. Then we see the only growing things, ashes "grow[ing] like wheat into ridges and hills," and ashes rising like chimneys, and "men who move dimly and already crumbling through the powdery air," suggesting a movement much like the sweeping ups and downs and fading away of a musical composition.
Occasionally. it says we see a slow line of cars which "crawl" and "creak," adding to the song of the ash heaps. There is a flurry of ashes caused by the digging of spades, a great cloud of dust, this amazing (and amusing) billboard rising transcendent from the dust. Then there is a sinking and a dimming and a brooding, all very evocative of an orchestral piece winding down to its coda.
Not sure if this is what you (or your teacher) is looking for, but the elements and the dust do create a veritable symphony of the ash heaps.