GIMP: Resizing image and filling the empty space

1. Resize the Canvas

Menu -> Image -> Canvas Size...

Note: Click the "Center" button to center the original image.

 

2.  Duplicate the layer

Menu -> Layer -> Duplicate Layer

3. Resize the lower layer

Select the lower lay (on the right panel)

Menu -> Layer -> Scale Layer...

Change the Width/Height to a bigger value to make the image cover the whole Canvas.

4. Fuzzy the lower layer

Select the lower lay (on the right panel)

Press Ctrl-A to select all on the lower layer

Filter -> Blur -> Gaussian Blur...

Adjust "Size X"/"Size Y" to blur the lower layer