I notice that most of my winter, aka “snow”, pictures have this dull gray color to them. For quick white balance correction in Gimp, open the image. Now from the menu on the Image, choose COLOR | AUTO | WHITE BALANCE

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Gimp’s auto white balance has been pretty reliable to quickly set the correct tones in my grayish photos. See the results here:

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And there you go – one click white balance correction in Gimp!