The options are:
Spectral tones - as described; dark tones are translated to blue, mids to red, highlights to green
Detail map - difference between norm of bilateral and image norm (bilateral params can't be chosen)
JPEG CbCr detail - YCbCr chroma channels equalized and displayed in greyscale
local equality - map of pixels with identical values to 8-connected neighbours
I use it to check for image defects because sometimes it can be hard to tell whether a filter I'm making has introduced artifacts, or it's an issue with the original. That's more an issue with compressed of course.
Back on the subject of PCA, I don't know of one already in G'MIC. The closest I know exists is k-means colour matching (-index and -autoindex), I'm not certain it could reduce to smooth greyscale. PCA could of course be implemented in a few ways, I can't promise to produce one any time soon. Maybe somebody else will have a go before me...