GIMP Paint Studio is a creative commons set of tool presets,
brush presets, color swatches, patterns, and config files that will
transform the GNU Image Manipulation Program into a robust painting and
materials-emulation application.  Great for painters, airbrushing and
re-touching, and illustrators.

Note that this will overwrite any config files you might already have
in ~/.gimp-2.8 so if you've got customizations, back 'em up.