I don't use Dundjinni or Gimp, but I do use Photoshop to do post-work on Campaign Cartographer images. CC3 isn't always very good at compositing, so when I needed to place some line drawings on a map, I exported the image from CC3, then used a raster editor to place the drawings. I also use it for labeling, since CC3 has a rather serious issue with aliasing (jaggies) on text.
Borders and other decorative elements are also sometimes better done in a raster program like Gimp or Photoshop. I'm not sure what Dundjinni's capabilities are, but I am sure that it does have its limitations, just like every other program does.