First off, that's a really cool looking continent. I always seem to round mine off too much instead of staying with these cooler shapes. Second, there are a ton of different ways to achieve that "fuzzy-edged" look. Using the eraser is one that I used to use most often but I now use masks (not sure if Gimp has masks). Making a selection and then giving it a fuzziness also works (again, not sure if Gimp has a fuzziness setting). As for making things semi-transparent you use the opacity function or if you want something a little different change the blend mode.