Reduce sky Gradient

A single swipe across the sky with the Quick Select tool makes an almost perfect selection.  I did have to remove a couple of tiny bits of the mountain peaks with a smaller cursor, hold down the Alt (Opt) key.

Invert the selection (Shift Ctrl I)  and copy to a new layer.

Make a new layer for the sky between background and the copied mountain peaks.

I've put some red behind the mountains to demonstrate how well this works.

My preferred method of selecting the new foreground and background colours, is to select the Brush tool, and Alt click the colours I need.  You'll  need to hit the X key to toggle foreground and background colours to sample the second colour.  You could click on the foreground colour to open the colour panel, but that takes more clicks.  We hate wasting our clicks.

OK you are good to go.  Select the Gradient tool, and make sure you have the first option selected, which is Foreground to Background.

Holding down the Shift key while you drag, will force it to be perfectly vertical.   When you release the mouse will control where the gradient falls.  Note it will fill the layer behind the copied mountains layer.

Learning the Gradient editor is a very useful skill, so never miss an opportunity to practice

Click on the gradient in the Options bar to open it.  Then click just beneath the gradient to bring up some new sliders. 

The way it works is that the Active colour point has a Black triangle above it (the one with the cyan box around it).  Other, non active, colour points will have grey triangles above them.

With a colour point active, select a colour either from the image or the Colour Swatches, and drag to fine tune.  It's actually a lot of fun.