1- Open the image that could have benefited from a gradation filter. (You want a darker sky, lighter foreground.)

2- Duplicate the layer. On the top layer, select the Add Layer Mask  at the bottom of the Layers pull down menu. (It looks like a gray square with a white circle inside.)

3- Open the Gradient Tool. Select Foreground to Transparent.

4- Select black as your Foreground color.

5- Click and drag the Gradient Tool from bottom to top. Experiment for best effect.

6- In the layer box select Multiply.

7- This gives you 100% more density in the unmasked area. For less density, change the transparency of the top layer to a lesser percentage than 100%. For a darker gradation simply duplicate the top layer! And again for even darker sky. Watch the top get darker.

8- Of course there are many variations. Circle gradient, wedge, bar, etc. Also contrast etc can also be played with in this same manner. Wow. What a tool. Gradient combined with mask.