This is a basic run through of how I make mountains in Gimp using an overlay layer. The same method can be used to provide light and shade to general maps and is something I use very regularly.

Let us say that you have an image with the colours that you want in the right place. Here I have an image with a grass texture blurred into brown and then light brown-grey to designate a mountain range:

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Now this looks very flat. To create an illusion of depth we need to all light and shade. One way to do this is to create a new layer and fill this with 50% grey (7f7f7f in html notation). Set the layer mode to overlay:

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Now take a large round brush and paint with pure white on the edge of the mountains towards the light and black on the areas away from the light. This will generate light and shade for you. I use a tablet, so I just allow the pressure to give me more or less light and shade. If you are using a mouse I would suggest that you set the opacity of the brush quite low and build up the areas of light and shade.

After doing this it's often useful to take the smudge tool and just smooth away any hard edges. Now, this is the overlay layer that I created for my mountains:

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This is the result of that layer set to overlay over the coloured layer I had before:
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Now obviously the colours underneath are important too. If I was doing this with more care then I'd try to texture the mountain properly rather than having flat brown as the colour.

This method requires a little care, and is most useful with a tablet. I'll show a slightly longer, but perhaps more foolproof method next.