I have a similar GIMP technique, and since yours in a PS only tutorial i hope i can add my GIMP version without stepping on any toes.
1) Open a new transparent layer for your Mountains.
2) Use the Pre-set Galaxy Brush (large, medium, or small, your choice)
3) Check the Fade-Out Box and set the brush color to White.
4) Dab the Brush where you want mountains, erasing ("eroding" ) the edges to your preferance.
5) Filters>Distorts>Emboss (Bumpmap works too but i dont notice a difference when using a white brush colour)
6) Done and it should look similar to this. All in All a VERY FAST style for map BUILDING, and still good enough for a finished result.