Thank you Slylok, that's a very nice stuff, but I think it isn't exactly the same technic used here.

I tried it and could get similar results. In GIMP, I used a soft (25% hardness) brush, with about 30% opacity with a very dark brown color (HTML 1D170E), with the tool paintbrush. Draw the mountain shadow with that, and erase for the lights. Also there are some smal angled lines (vertical lines, 45º counter-clockwise) painted on a layer above with low opacity (also dark brown). Set both layers mode to multiply.

This way you can get mountains similar to what you want, and you can do everything with your wacom.

Hope it works for you as it worked for me.