This commercial add-on syncs editing in Blender and the Gimp. Supported versions are Blender 2.63 and GIMP 2.8.
Orest B writes:
Ever try creating a 3D object in Blender, and painting a texture for it in GIMP? This addon lets you paint in GIMP while the texture updates automatically in Blender, or switch to painting in Blender and see the texture update in GIMP. It also automatically imports the UV layout from Blender into GIMP, and adds shortcuts to GIMP to save and reload the texture without affecting certain layers. [here] is a video demo, and the manual has more info on how to use it. Right now we’re trying to make some cash :) and are selling the addon for $5, we plan to make it free eventually.