Okay, so as promised, here's the second method.
As before the image will have a base layer with the colours on it, and an overlay layer that generates the light and shade. This post (and the couple that follow it) will show how to generate a decent overlay layer using filters rather than drawing it in by hand.
First create a new transparent layer using the layers dialogue, or Layers->New Layer (shift-ctrl-n). On this layer use the lasoo tool to select the border of your mountain shape. It doesn't have to be exact. Here's mine:
Now we want to use gradient fill to fill this with a gradient that goes from white along the ridge lines to black along the edges. Here's the settings I use for my gradient settings. I've highlighted the important ones - especially Shape: Shaped(angular):
Now click and drag a line from the middle of your selection to one of the edges of the selection. This should fill the selection with your gradient. Now invert the selection (ctrl-I), grow the selection by 1 pixel (Select->Grow selection) and fill with black, you should have something like this:
This will be the layer we use as a bump map.
Next, create a new layer and fill it with 50% grey. This will be our overlay layer. Go to Filters->Map->Bump Map. In this dialogue select your gradient filled layer as the map in the drop down menu. Play with the depth setting to get a reasonably dark shadow and make sure you select the angle for the azimuth so that you have the light coming from the right direction:
When you are happy with the preview, hit okay and you should get something that looks a little like this: