I'm not an expert on terrain masks, but hopefully, this (http://www.feral-heart.com/index.php?option=com_content&view=article&id=7&Itemid=10) will be useful. I think I used that when I made my first map. :)
At the top of the Gimp window, click on the 'Image' menu, go to 'Mode' then click 'RGB'. :)