The gray and white checkerboard on both the gradient and the fractal layer indicates areas of 

transparency.

G     

To see how this works, change the Merge Mode of the Box Trap layer to Normal and then 

Shift Click its Visible icon to toggle the other layers on and off. 

Notice that the underneath layers are only visible in the checkerboarded area of the top layer.

Next: 

Fine tuning the gradient

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