If you have a big flat area it is because you left a big spot all white. So yes, you have to adjust the curves down until you get just a small peak if that is what you want. I'm not exactly sure what the reference to ridges is, but the noise generator included with GIMP leaves diagonal striations. Use Felimage to get away from that. If you want to alter the color of your mountains you need to make a new color layer and mask it to fit your mountain shape. If the mountains are essentially a 50% gray bump map/embossed layer set on overlay it will show the color of the layer underneath, whatever it is.