This is going to be tough. The fact that The Great Gatsby is considered to be a landmark of Western literature is a part of its difficulty. No matter what, any summary of fifty words is going to leave out essential parts and aspects. I am not sure it is possible to encapsulate its greatness and meaning in only fifty words. With this in mind, I think that a summary would have to focus only on Jay Gatsby:
Jay Gatsby is rich. He is in love with the idea that he can make Daisy love him. Gatsby represents new money, while Daisy represents the establishment. Gatsby uses his materialism to influence Daisy. His tale represents the frailty of wealth and popular social acceptance, and the emptiness within both.
With this summary, you lose Nick, Tom, Myrtle, George and Jordan. I am not sure how you can incorporate these essential characters with the fifty word limit. Being able to capture the set up between "old money" and "new money" drove the summary. The emptiness element was another aspect I wanted to include. However, I completely concede that there are some essential elements that are missing when you add the fifty word limit. However, within the structure that seems powerful and strong, there is a frailty, which I think is essential to both the summary and work in general.