Jay Gatsby's dream to be with Daisy Buchanan is the entire driving force of the novel. It motivates him to become wealthy, move to New York, and throw parties in hopes that she will attend. The Great Gatsby completely revolves around love and is the motivation behind many characters' actions. Even Nick Carraway and Jordan Baker help with the pursuit of Daisy by being Gatsby's wingman. Gatsby's entire life is centered on loving and pleasing Daisy and is the cause for every action. He loved Daisy so greatly that he took the blame from her, consequently getting himself killed.
The Great Gatsby isn't just about love. There are other themes as well, such as wealth, the American dream, and social status. Gatsby's love for Daisy may be important to the plot, but there are other situations occurring simultaneously. The novel also goes into detail about other characters' lives, such as Tom Buchanan and Nick Carraway.
[P1] Jay Gatsby is in love with Daisy Buchanan. [P2] The plot of The Great Gatsby is motivated by love. [P3] Therefore, the theme of The Great Gatsby is love.