Here's two tutorials I used to turn a color photo taken on a sunny day into a darker goth style image using Gimp. This is the original photo taken in a cemetery on a fairly sunny day.
Using the tutorial found at the Gimp website. I changed this to this.
I also made some slight adjustments using Colors/brightness-contrast.Then using the tutorial found at gimpguru. I added some fog.
The entire process took under 20 minutes, and changed a bright sunny day into a darker Gothic style seen.