Iain Fergusson’s “Noise reduction 5” seems to fix skin grain in a dark room - see sample and the applied filter result below. My question is: why does this work?
If someone has written something like Grokking the GIMP by Carey Bunks for G’MIC explaining how this works, a link would be appreciated. I would really like to understand what I am doing rather than just doing it. 
have you seen this page : Iain’s Noise Reduction for G’MIC and GIMP | Iain Fergusson: Christchurch Event Photographer ?
it could give some insights.
Also I’m sure @Iain could help !
No, I have not - thank you, David!
The article you linked to pointed me in the right direction, and I found http://www.cambridgeincolour.com/tutorials/image-noise-2.htm , which gave me the background understanding I needed. Thank you again!