Fill 0% on the layers.
Blending Options-Gradient Overlay



Gradient’s parameters:



For your convenience now we should merge down in a group all the layers composing the penguin’s foot.
Make a copy of this group then and apply Free Transform selection to turn around the group on 180 degrees vertically.



Now we have to put together in a group both groups of the feet.
Make a copy of the new one and use again the Free Transform selection to turn it around and compress it also vertically.



Select after the last operation a brush having blurred edges and grey color and apply it to create on the picture the effect of flatness.