How interesting, I found a plugin for Gimp that allows you to use layer styles as Potoshop, Layer Effects.
The Python version (layerfx.py) allows preview.
My second map:
No countries, no rivers.
I make it with Photoshop using Ascension's Atlas Tutorial.
In Creating the Continental Shelf, Select > Color range = black with a fuzziness of 150, I use 50 instead.