Does anyone know anything about how "money can't buy happiness" in this novel? Any quotes or insights?
Concerning The Great Gatsby, you should be careful of looking for simple, easy one-liners in sophisticated fiction. Human existence is complex, not simple, and sophisticated fiction usually reflects that.
For instance, in this novel, money is only a means to an end for Gatsby. He doesn't strive to become wealthy because of greed. He strives to become wealthy for the sake of winning Daisy back. Money only matters to Gatsby because he thinks it will help him win Daisy back. An easy one-liner about money not buying happiness doesn't apply to Gatsby. Daisy will bring happiness to Gatsby, nothing else. That is the issue.
For another example, Tom is definitely leading a happy life, so to speak. He has a beautiful wife, doesn't have to work as far as the reader knows, and has a girlfriend, too. And he wins in the end. He is so sure of himself and so ignorant that he thinks he's always right and enjoys the winning. He has what he wants and gets what he wants. If one wants to apply one-liners to the novel, one could argue that the novel suggests that money can buy happiness. A reader's judgments concerning Tom do not take away from the fact that Tom is happy with his situation and his life.
The corruption of the American Dream is at issue in the novel, as are one's ability or inability to recapture the past, illusion, and other issues. But saying that the novel shows that money can't buy happiness is too moralistic and too easy and too simplistic.
If you think about it, you can argue that this whole novel is about how money cannot buy happiness.
Gatsby has wanted to be rich all his life. And then since Daisy rejected him he has wanted all the more to be rich. He thinks that he will get money and then Daisy will want him and he will be happy -- money will buy him happiness.
But then he gets the money and he does not become happy. He sees Daisy again and he seems to have a chance with her but ultimately she rejects him. His money has not bought him happiness.