I discovered this pencil drawing effect by accident while messing around with G'MIC's rodilius filter. You will need the latest G'MIC for this tutorial. The photo I'm using is this one.

http://www.flickr.com/photos/8765199@N07/4124918919/

Open your photo and duplicate the layer twice.
On the top layer, run G'MIC/Artistic/Rodilius. I used default settings but as with all tutorials, feel free to play.
Change the layer mode to Grain Merge.



Merge the layer down and desaturate it.
Add a new layer and bucket fill it with 50% grey.



Go to Filters/Noise/HSV Noise and push all sliders to max and Holdness to min and click on OK.



Use Colours/Desaturate on the layer and change the layer mode to Grain Merge, reducing the opacity until you have the effect you want.



Duplicate the background layer and push to the top of the layer stack.
Run G'MIC/Black And White/Hard Sketch and change the layer mode to Multiply.



For a colour version, make the background layer invisible, select Image/Merge Visible Layers, make the background visible again then change the layer mode for the top layer to Value.

Instead of rodilius, I put the top layer into Dodge mode then merged down and desaturated as usual.