Jay Gatsby's death is significant as it symbolizes the demise of the American dream. The American dream purports that anyone, regardless of their birth status, can achieve upward mobility through hard work.
Ultimately, Gatsby's death eliminates this dream for all of the characters. Specifically, Nick loses his bond with the upper class and Daisy loses her opportunity to find true love. You.may continue to analyze each character and discuss how this loss affects them. If you'd like more help, please contact me. ;-)