Thank you very much.I don't use GIMP, but I might be able to help. What kind of effect are you trying to achieve? Do you have an example?
My goal is the map of a continent with top down mountains and large forests. Over that, I want to place several layers to show the borders of the continent during different ages.
I just found this tutorial ( http://www.fantasticmaps.com/2012/01/a-week-of-mapping-tips/ ) and want to try to adapt it to GIMP in the next time.
I want to post two/three examples (of my own cartography) soon, but I am to tired/stressed at the moment.