I took an interest in the movie Gone Girl because the book's author is from my home area, and a graduate of my university. I didn't actually read the book, so my thoughts are purely based on the story as a movie.
What Gone Girl shows is the unreliability of humans, especially together. Amy is invested in being a step ahead, something lacking in her life. The childhood books based on her life, Amazing Amy, always show a character succeeding, in control. Transplanted from New York to Missouri with her husband, mostly out of money, in a society she never experienced, and now confronted with her husband's affair, she finds a way out, by staging her disappearance and framing Nick for her apparent murder. Nick is invested in himself too, seemingly more attached to his twin sister than his wife, pursuing the aforementioned affair, and using Amy's trust for his own interests, like operating a bar. The movie, and I assume the book, starts with the audience feeling one way about Nick, then provides the clues to Amy's personality toward the middle.
As incredible as the story will seem, the closure is reality. The manipulator always wins. The cat-and-mouse game likely continues, but the better manipulator will inevitably triumph.