Just a mention really concerning this software, It is often said that Gimp has a steep learning curve, because it has so many tools this is to be expected, personally I think that the best way of getting to grips with Gimp is to search for tutorials on the Internet if you are unsure “How To” something, I certainly have, and have always found what has been looking for.
I use Gimp for image manipulation, which can also be part of my workflow, I tend to be “old school” and try to get it right in the camera thus do not really do much in the way of manipulation but on the odd occasion I will have to use software to clone out details that should not have been part of an image, or perhaps “beautify” a portrait and that is where Gimp comes into the picture. The clone and healing tools are the ones I tend to use most, (don’t think any explanation necessary) also useful are the image resizing tools, I can do this in Darktable but prefer Gimp because it gives me a wider choice in how the resizing can be done (canvas size, print size, scale image etc.) finally it should be mentioned that there are plugins for Gimp to do almost anything, I found one called Sharpen (Smart Redux) that has a multi stage method of sharpening an image, lots of settings that can be tweaked but even default values can do wonders on the right kind of image, I found it worked particularly well on certain B&W images.
Gimp is of course useful for other graphics work, the header image for the Workflow category was made using Gimp and it was also used for modifying and manipulating the other header images.