Inside GIMP, Colors -> Levels Click on the "Pick gray point" eyedropper (the middle eyedropper) and then click on a pixel in the image. Then go change to the Red/Green/Blue channels and you'll see ...
I have recently calibrated and profiled my screen, with a ColorHug device. I'm using Gnome, and have the profile installed through gcm. When opening an image in GIMP and DigiKam, the results are ...