In reference to Gatsby's parties, why might it be said that his life was an illusion with no substance?

Gatsby's parties were events.... nothing more. The people who attended were strangers, the atmosphere was planned, and for Gatsby, the evening meant nothing more than the means to an end. In this, we can see the illusion with which he surrounded himself. But is was only an illusion, it wasn't Gatsby. Thus, there was no substance.