Sorry so late to the post, I found ZombieNirvana's guide and was interpreting it for Gimp, while attempting to learn Gimp (I have been a moderate user of Campaign Cartographer, but am trying to grow into using/making my own styles). I got to the point where he uses 'Filters > Brush Strokes > Sprayed Strokes' in Photoshop to make the rough edge of the parchment background and seeking a Gimp equivalent, found your adaptation on the web. I was disappointed to see that you also got to that point and couldn't find the answer. This set me to experimenting with different filters to try to get something that looked good. I was able to use Filters->Distorts->Ripple to good effect and decided to come over here and let you know. By this time, I'm sure you've moved on, but perhaps another experimenting soul will find this on their map-making journey and find an answer.