What is the theme of the great gatsby?

Like any novel,  there are many themes in the Great Gatsby. 
One of the most prominent themes in the novel, as I saw it, was the theme of dreams. We can see this theme through the green light at the end of Daisy's dock and how Gatsby dreams to be with her. We can see it through how George Wilson dreams of escaping his impoverished lifestyle and clings to Tom's car like its his lifeline. There is also a question that is posed in the novel - is it worth it to achieve your dreams? Once Gatsby had Daisy, the glitz and glamour of his dreams faded away. Perhaps she was more enticing when she was just out of his reach. Within this, there is also the theme of the American Dream, the white picket fence. Fitzgerald comments on how the American Dream becomes like a mess of material excess. He comments on the greed and unnecessary lavishness of the time with the juxtaposition of Gatsby's beautiful mansion and the valley of ashes. 

There are plenty more themes within the novel, but these were the ones that stood out to me. You could also explore the hypocrisy of our narrator, Nick, or how cold the upper class appear to be. Hope this helps.