@afre, honestly I didn’t waste one thought on the desaturation method in this example.
The first thing I do in any Gimp processing is always duplicate the image and use:
colors → desaturate → desaturate (mode:lightness)
This gives me the layer for my contrast modifications and also gives me the so called L mask. I generally just use this one luminosity mask and and select the regions I want using curves.
Then for the c2g I did:
colors → desaturate → color to gray
I just used the standard setting. The result is totally over the top. That is why I mask it with the above mentioned luminosity mask and select the intensity of the c2g layer for each luminosity region using the curves on the mask.
Really fast and simple. 
@patdavid, yes it is very easy to overdo…and thanks for the nice words.