Expanding the sky in panoramic photos

Useful for changing the aspect ratio of panoramic photos, which are usually much more wide than they are tall - How to expand sky in any photo with GIMP - YouTube

Scale up a copy on a new layer.
Create layer mask on original layer.
Select the layer mask in the layers window to select it for editing.
Gradient black to white Foreground to transparent to blend in the orig with the scaled up sky.

What I did for the Tokyo pan was, since the sky was just blue,
did a rectangular selection of only sky,
copied the selection, pasted to new layer
and for that layer, Layer -> resize layer, expand upwards.