The main themes include deception. Both Nick and Amy deceive each other in different ways throughout the story. There's also the theme of revenge. Amy's elaborate plan to get back at Nick is a central part of the plot. And the theme of perception versus reality, as what the characters believe and what is actually true are often very different in the Gone Girl novel.
In the Gone Girl novel, a major theme is the complex nature of love. Their love turns into something dark and twisted. Additionally, the theme of power dynamics in a relationship is there. Amy often seems to have the upper hand in manipulating the situation and Nick. And the theme of public vs private lives, as their private problems are made very public through the media.
One of the main themes is the breakdown of a marriage. It shows how the relationship between Nick and Amy deteriorates over time. Another theme is the media's influence. The way the media sensationalizes their story and affects public perception is a big part of the novel. Also, there's the theme of identity, as both characters present different versions of themselves to the world and to each other.