I was not so far out with my dumb deleted entry 
That video has every tool including the kitchen sink in it. I can not see anything wrong in your screenshot.
EDIT Oh yes, see it now, you have a white base layer. Turn visibility off temporarily. However for the gradient.
First add a layer mask Layer -> Mask -> Add layer mask - White (full opacity) . That has to be active so click in the mask icon. Active has a white border not easy to see with the white mask but it is there.
As it happens, it does not matter if the gradient type is RGB or HSV but if you want choose HSV, and Shape = Linear
Foreground / Background is default black / white so the gradient goes from black to white. Anything black in the layer mask is transparent so you need to go white (bottom of image) to black (top of image) If you apply the gradient bottom to top then you need to reverse the gradient. There is a little icon top of the gradient tool options for this.
Now apply the gradient on the image to blank out above the clouds. Hit the enter key to complete.
example: https://i.imgur.com/JJkbnsi.mp4
That video has every tool including the kitchen sink in it. I can not see anything wrong in your screenshot. EDIT Oh yes, see it now, you have a white base layer. Turn visibility off temporarily. However for the gradient.
First add a layer mask Layer -> Mask -> Add layer mask - White (full opacity) . That has to be active so click in the mask icon. Active has a white border not easy to see with the white mask but it is there.
As it happens, it does not matter if the gradient type is RGB or HSV but if you want choose HSV, and Shape = Linear
Foreground / Background is default black / white so the gradient goes from black to white. Anything black in the layer mask is transparent so you need to go white (bottom of image) to black (top of image) If you apply the gradient bottom to top then you need to reverse the gradient. There is a little icon top of the gradient tool options for this.
Now apply the gradient on the image to blank out above the clouds. Hit the enter key to complete.
example: https://i.imgur.com/JJkbnsi.mp4