There are lots of rumors going around about Gatsby. Some people say he is a spy and others say he once killed someone. The truth is a bit simpler than that: Gatsby was born into a poor farming family. He had to work to put himself through college.
These rumors, as well as Gatsby's reality, illustrate the theme of illusion vs. reality. In this world, nothing is as it appears to be. Everything is an illusion -- everyone is pretending to be something they're not.
In this, Gatsby is no different. Many characters, Gatsby included, wear a mask that hides the real person underneath.
The author is making the point that the world of glitz and glamour is often all for show.
