Nice to see G'MIC's smart colorize algorithm making its way into core GIMP. The research team worked with David Revoy, the character designer of several Blender open movies (such as Sintel or Spring), so I've heard a lot about it from him. He's written a few blog posts (e.g. [1]) and videos on how to use the Krita and GIMP implementations of the feature.
[1] https://www.davidrevoy.com/article324/smart-coloring-preview...