Using rumor to tell Jay Gatsby's story adds to the mystery and enhances the drama. Fitzgerald depicts him as some kind of fantasy character from the outset. The fact that the narrator, Nick Carraway, states that Jay was in search of the "holy grail" alludes to the fairy-tale nature of Jay's life. Most of what is known about King Arthur and the Knights of the Round Table and the search for the holy grail is based on legend and rumor, just as in Jay's situation.
Furthermore, Fitzgerald wishes to depict the nature of the American Dream. It is based on the idealistic notion that one can achieve whatever one wishes should one set goals and work hard at reaching them. This, however, is based only partly on concrete evidence but is mostly also the result of rumor and speculation.
Revealing what is known about Jay Gatsby through rumor creates intrigue not only for the narrator, Nick Carraway, but also for the reader. We are drawn into a journey of discovery and, like Nick, are constantly provided little glimpses into the reality of Jay Gatsby's life. We are encouraged to seek more and get to the bottom of who or what he actually is--what drives him. Jay Gatsby is like a fairy-tale prince on a medieval quest. He wants to save his princess from the clutches of the evil ogre, represented by Tom Buchanan.
It should be apparent that if we were presented with all the cold, hard facts about Jay, we, the readers, as well as Nick Carraway, would soon have lost interest. As such, the story would have been bland and should have comprised only a few pages of interest--a short story. In contrast, we, together with Nick, are enthralled by a character who wants to recreate the past and who refuses to give up on his dream. We want to learn whether the stories about how he achieved success are true and if the rumormongers are accurate in what they say.
Furthermore, Jay's antagonist, Tom Buchanan, thrives on these rumours and they are part of the arsenal he uses in his attempts to discredit our hero. We, like Nick, want "to get up and slap him on the back" when Jay counters Tom's attacks during their confrontation. We are similarly disillusioned when we discover that Daisy
...was drawing further and further into herself...and only the dead dream fought on as the afternoon slipped away, trying to touch what was no longer tangible, struggling unhappily, undespairingly, toward that lost voice across the room....
Fitzgerald leads us through the drama, and we are disappointed and regretful when it is revealed that many of the rumors about Jay are, indeed, true. We learn about his association with characters from the underworld, such as Meyer Wolfsheim, and feel pity that our hero should have allowed himself to be tainted by corruption. We regret that the purity of his dream has been tarnished by vice.
In the end, though, in spite of the scandalmongering, the rumors, and the hard truths, Fitzgerald still leaves us satisfied knowing that, even though Jay's dream fails, he has at least tried. With the little he had been given, maybe the choices he has made were ones he took as a last resort. There is no other way he could ever have hoped to find his holy grail.