I have a feeling that if Gatsby were written today, things might go something like this:
Dear Mr. Fitzgerald,
Thank you for your interest in Famous Literary Agency, however I'm afraid your story just isn't right for us.
As I'm sure you're aware, a 50,000-word story hardly qualifies as a novel. In addition, I'm afraid the market is currently saturated with stories of self-involved young people.
Have you considered making Gatsby a vampire? It would make the work far more saleable and not require much of a rewrite on your part, because in addition to his shadowy past, he does mostly come out at night.
I confess as well to being perplexed by the title. Perhaps in an alternate universe, schoolchildren write essays about what makes him so "great," but I'm afraid our business takes place in the real world.
Famous Literary Agent
PS: Lose the billboard.