Using the GIMP, created a new white layer, setting opacity at about 90%. Then made a bilinear black to white custom gradient and touched up a few key spots with the eyedropper and paintbrush. Hid the layer, activated the background, and used the Focus Blur plugin (better than a gaussian blur) with the gradient layer as a depthmap and a radius of 10. Touched up curves and saturation.
Compare with the original.