Now a choice has to be made as to how to add the flood fill into the sky area.
In First Version "Dusk" I added the flood fill onto the background layer
and adjusted the transparency in the flood fill options.  I just used the undo feature 
and readjusted the transparency until I got it like I wanted, transparent enough 
so the cloud forms and finest branches would still show through. 

In the Second Version "Dawn" I promoted the selection to a layer, did the flood fill, 
and then adjusted the transparency in the layer palette.

Selections / Promote to Layer

Each method gave slightly different results.

"Dusk"

"Dawn"

Select  gradient in the styles option box and select the "Flood Fill" tool.
The gradient is the same in both versions except that in one I selected "Invert Gradient"

If you opted to promote the selection to a layer for the flood fill, you will now need to 
merge the promoted selection with the background layer for the next two steps to work.

Layers / Merge / Merge All