Here is one tutorial that is pretty simple--http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=13363.0
There is also this one, but I don't believe everything in here would be possible in GIMP--http://sugar-hype99.deviantart.com/art/How-To-Get-6-Terrains-on-FH-259394554
Hope this helps! ^^