It makes absolutely no sense; and I think if you stopped to think about it you would see that.
To what kind of layer will this C-shaped mask be applied?
You keep saying "gradient" but your screenshots are of Gradient Map layers, which are a very different thing.
So I need you to explain the situation further. Why are you using Gradient Map layers?
Oh crap, not the Digital Dog.
Ignore anything about placing reading points on the skin. This is too rigid. Only take readings by moving your mouse around.