An interesting article on the case that inspired the film. I had no knowledge of it, but found the film very interesting.

And yes, whilst I found the overall tone of the film to be one of profound sadness, I did find it occasionally darkly funny, though this is clearly a drama, not a comedy (nor has it been marketed as such, as far as I can tell).

For example, the undercurrent of dark humour is present in the scene when Natalie Portman's character visits the pet shop for "research", and the owner tells her not to touch the "bait", before she then simulates the sexual encounter to uncomfortably amusing effect. Not funny? Well, perhaps not for everyone. It's like that meme with Stalin: Dark humour is like food. Not everyone gets it.

I also remember stifling my giggles when I went to see Gone Girl in the cinema, especially during the last third. No one else was laughing. But then I saw an interview with Gillian Flynn, who wrote the screenplay for Gone Girl (adapting her own novel) and she said she deliberately made the film version funnier, with lots of dark humour. So I guess it just went over everyone else's head at the screening I attended.

I think there is often dark comedy to be found in dark situations, in my experience (for example, I could tell you a hilarious story about something that happened at my father's funeral, despite it being one of the most miserable days of my life). Then again, I appreciate that's based on personality and temperament. If something makes you laugh, it's funny for you. If it doesn't, it isn't for you. :)

My review of the film, if you're interested: https://medium.com/simon-dillon-cinema/film-review-may-december-142601477ad9