Adding Gradient to Layer Mask

There is an advanced technique for replacing a drab sky. It requires a picture of a nice blue sky with clouds, nothing else on it. You can take it youself or obtain one from another source, e.g. Google Images.

1. Open the picture with the "good" replacement sky and copy it to the clipboard

2. Open the the picture with the drab sky and paste the replacemet sky. It will come in on a separate layer. Activate the move tool and with the corner handles resize the sky and drag it over the drab sky. Set the layer blending mode to darken.

3. Set your foreground color chip to white

4. Drag the foreground to transparent linear gradient from below up (south to north) to the top of the tree line, while holding down the shift key

5. With a soft white brush at reduced opacity paint the horizon a tad, as this should be lighter than the rest of the sky.

Or  . .  . it would be better to forget about the mask. 

• Select the sky using the magic wand tool.
• Copy the selection to a new layer (Ctrl+J). 
• Lock the transparent pixels on the new sky layer (click smaller padlock icon above layers panel)
• Select the sky layer and apply the gradient

I think I need some morning coffee. 

Sorry, I've totally messed up what you should do.  The blue-white gradient needs to be applied to a background (or lower) layer and the sky mask will allow the gradient to show through in the sky area (unless you masked the ground).  Sorry for confusing you but it would be helpful if you show us what you have done and we can figure it out better for you.

Can you give us a screenshot of your layer panel.  I'm guessing that you either do not have the correct layer selected or you need to make sure the sky mask is selected before applying the gradient.