What symbol does Fitzgerald use as the outward manifestation of Gatsby's wealth? What does this reinforce?

Actually, there are several symbols of Gatsby's wealth in the novel, and each serves to reinforce ideas developed within the novel as a whole.

First, Gatsby's mansion symbolizes his great wealth. Grand and quite beautiful, it features the best furnishings that money can buy. As a symbol, however, it functions on a deeper level. Gatsby's enormous house is filled with empty rooms, except for the hours when they are filled with strangers and free loaders. The emptiness of his home and the sounds of footsteps echoing through hollow spaces is very suggestive of his life. Furthermore, the house itself is as imitative as Gatsby himself. Its design is not original:

. . . it was a factual imitation of some Hotel de Ville in Normandy, with a tower on one side, spanking new under a thin beard of raw ivy . . . .

Gatsby's house is an imitation of a hotel, which itself was an imitation of a castle. The house is so new that the ivy has not had time to grow and thicken. Gatsby is like his house: a new creation developed through imitation.

Also, Gatsby's yellow touring car is a testament to his wealth. Big and beautiful, it shines, reflecting light from its glass and its metallic fittings. It symbolizes the excesses of great wealth, but in its gaudiness, it also symbolizes the conspicuous display of wealth associated with "new money" in the novel. Tom Buchanan calls it "a circus wagon," demonstrating what Fitzgerald develops as the arrogance and sense of superiority of "old money." Tom judges Gatsby as being crude and tasteless, feeling nothing but contempt for him, despite his money.

Finally, Gatsby's beautiful shirts that he tosses into the air for Daisy symbolize his wealth and excess. That she would respond to this display so emotionally suggests her own superficiality.