This is pretty much what I did step-by-step. Keep in mind that the GMIC settings need to be adjusted differently for each image ... a set list of settings just won't work.
It's good to have a subject that contrasts with the background some.
Original image
hereOpen the image, duplicate the layers twice (so you have a backup one on hand), I turned off the visibility of the bottom layer. Why? Don't know. Doesn't make a difference in the results.
Top layer: Colors > Desaturate, use Luminosity setting.
If there isn't a lot of contrast, go to Colors > Brightness and Contrast and adjust so that you have more contrast. Don't get too bright. Be sparing with the settings. I increased the contrast to about 30, decreased the brightness close to the same.
I scaled the image to 50% in these to save room.
Filters > G'MIC > Artistic > B&W Hard Sketch.
Play with the settings in there for your image. Every image will have different settings. Try a few.
Set that layer to Grain Merge mode.
If you don't like your results, do Undo to restore and start again.
This image didn't come out as nicely but I don't think it's the filter's fault. Hopefully this will get you started using the filter and you will feel more at home with it.
You may even want to try the filter on the original image. Just be sure to duplicate the original layer first.
You got a brilliant idea and lovely results...may even be the starting point for a new artistic filter
PS please could you delete the message above this ?..i did a accidental click ,sorry