Made in GIMP 2.8 running on Ubuntu 12.04.

Brush used: artistic.gih by Chrisdesign (http://chrisdesign.deviantart.com/)

Layers used:
1. Gradient (radial, light gray to dark gray)
2. artistic.gih covering layer (set to 'divide')
3. Duplicate of layer 2 with canvas applied
4. Duplicate of layer 3 set to 'soft light', with a layer mask containing a gradient (radial, white to black)