There is a parameter missing from both the FS9 and FSX G500 GPS scripts that will add a "terrain shadowing" effect to the color terrain map view.
I have discovered that there is one entry to "add" and another to "modify" in order for this to work:
Open the default gps_500.xml script file in notepad.exe and add the entry <TerrainShadow>True</TerrainShadow> entry just above the closing </CustomDraw> entry.
Edit the <LayerVehicles>0</LayerVehicles> entry from 0 to 1 as shown above. You should now have a color terrain map that looks like this:
Shot at 2009-10-02