This is a good question. There are many levels of illusion in the book.
First, the most obvious example of an illusion or an illusionist is Jay Gatz, who reinvented himself as Jay Gatsby. He is really a poor North Dakota farm boy, who pursued the American dream and got it. He reinvented himself and gave the illusion of old money, sophistication, and worldly pomp. The irony is that people were entertained, but no one really believed in him.
Second, on another level, the title of the book lends itself to an illusion. The "Great" Gatsby is the title of an illusionist, which is incredibly apt, because he was able to make himself into something else. Moreover, his parties, which were over the top also had the feeling of an illusion. It can also be argued that his guest, who did not know him at all, projected him to be something - whatever their imaginations concocted, which is another form of an illusion.
Third, Gatsby was not alone in his illusion making. Other characters, apart from Nick, were also illusionists. Even Daisy concealed her true feelings for Gatsby many times.
In short, the deeper you go, there is little reality in the book. It is all smoke and mirrors.