I have experimented with several ways of targeting “sky” areas for smoothing in my cloud/sky photographs. I sometimes try selecting by color (shades of blue/cyan) and other times tinker with luminosity masks. The goal is minimum digital noise in these areas and good results with anisotropic/bilateral/thin brush. Am I overlooking a better approach? The docs I found on G’MIC smoothing seem deeply technical.
There are a good amount of smoothing filters in gmic. Two of the more popular for skin retouching, which yield a similar result as what you’re looking for, are dream smoothing and bilateral smooth.
Of course, you can check them out for yourself!
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This reminds me this video :
Here, I used the G’MIC select-by-color filter to mask the sky and change its color, but I guess you can just blur it instead after the selection.
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The G’MIC interactive color masking is slow even on my fast PC, but a few minutes for a way better mask than other tools. Great interface and great results - thanks for the tip!
Yes it is slow indeed 
Multiple cores help a lot for this filter !