The quote that sums up the themes of both novels could be from either novel.
From The Adventures of Huckleberry Finn by Mark Twain, an ideal quote summarizing the theme of an outsider in his society who tries to make sense of the hypocrisy would be this one, when Huck gets annoyed with his constant internal struggle on whether or not to turn in Jim the slave.
I was a-trembling, because I'd got to decide, forever, betwixt two things, and I knowed it. I studied a minute, sort of holding my breath, and then says to myself: “All right, then, I'll go to hell”—and tore it up. (ch 31)
Huck decides that if not turning Jim in means he will go to Hell, then he is going to go to Hell. He makes the choice to do the opposite of what society wants or tells him to do. He chooses to follow his real conscience, the one in his heart that tells him right from wrong—because he has finally realized that this is not the same thing as what is legal or what is considered moral. Sometimes the individual can be right and society can be wrong.
Gatsby comes to this same realization. He faces an untenable situation too. He has tried to make something of himself, but he is not really happy. He longs for what he can’t have. He wants a real human connection. He wants to be in love with Daisy, and have her show him love in return. He wants his fantasy to show itself as reality.
Another quote that you can also use is from The Great Gatsby. It speaks to American individuality.
"Americans, while occasionally willing to be serfs, have always been obstinate about being peasantry." (ch 5)
Americans have a love-hate relationship with money. It is an intricate part of the American dream. While we want nothing else but to be rich, there is nothing more we despise. Huck realized this in his travels along the river, particularly in the incident with the king and the duke. Americans will work hard for the rich, but do not want to be looked down on by them.