Cinepaint is an Open Source image editor developed for the movie industry specifically to support high dynamic range image editing.
Although originally based on the gimp image editor (for a long time cinepaint was called Film Gimp), this application has since diverged significantly.
This project was not very active in the last years.
There is a thorough cinepaint tutorial on manipulating 16bit photos.
Like the gimp, cinepaint needs to be tuned for memory usage when working with large images.