Could I use GIMP to edit a fractal continent?
GIMP is a raster image editor, if I understand correctly. If your "fractal continent" is in the form of a raster image, then yes, GIMP would work for that. If your fractal continent is being generated by a piece of software such as Fractal Terrains, then you would need to convert your file into an image to work with it in GIMP (this conversion would likely cost you access to the itneresting features of the original program that generated the continent).
More detail about the program that you're using (or even an example image) would make it easier to answer your question.