I gave it a try on GIMP 2.8. Some of the things that are different:

  1. "Grungy" brushes: you have to do a search and find some good brushes for GIMP
  2. I generated the background using the Plasma Cloud/Desaturate/Grain extract technique to generate parchment like things.
  3. Layer Groups: They exist, but you can't apply a mask to it, so I ended skipping that and adding a mask to each of the new layers
  4. Jitter and random sizes are possible, just create a new brush dynamic and use random size, color and etc
  5. Pressing enter to stroke a path isn't a thing on GIMP, but you can go to the path dialog and select "Stroke". If you click the option to use a brush, it will use the current brush settings (so you can set the brush options and then stroke the path)
  6. Adjustment layers don't exist on GIMP. I used a technique I saw on a blog: create a duplicate, desaturate it and change the opacity to apply it... Works, more or less.


This is the result:



Probably can use a better brush to do the shading, but I was satisfied by the first try. If someone wants I can post the .xcf file somewhere.