Gatsby represents the American dream of self-made wealth and happiness, the spirit of youth and resourcefulness, and the ability to make something of one's self despite one's origins. (eNotes)
Gatsby achieves great wealth and, to some extent, power and rises far from his humble origins in the Midwest. Despite this rise, Gatsby still feels unfulfilled. He needs more than what he has. This can be seen as a failure or as the American Dream failing him.
If the American Dream suggests that rising to wealth and power will satisfy one's ambitions and bring happiness, clearly this dream has failed Gatsby as it has failed to bring him happiness. He needs more than the material things he has gained in order to truly be fulfilled.
Regardless of the size of his house, the size of his car or the size of his bank account, Gatsby remains unsatisfied. He still needs Daisy - just like he did before he gained his material wealth. She eludes him and represents something much more difficult to obtain but still a kind of wealth.
“Her voice is full of money,” [Gatsby] said suddenly.
She represents wealth to him, but this is a different kind of wealth- an immaterial wealth that the American Dream cannot provide.