I never use GIMP actively but i think GIMP is more focused un image edition, more like Photoshop. Darktable is focused in processing RAW image files (and probaly even more things), applying color profiles, filters, retouching the RAW data, etc.

Because i never edit images i don't use GIMP (also due that i don't really learned how to use it). But i use Darktable to process my RAW photos taken from a camera, it just enhances the photos, with color correction, filters and other settings.