No matter how it was created, with the resulting gradient selection active
we create a Curves Adjustment Layer. The top right end of the curve line
represents pure white, the lower left end being black. When the line is
straight, as it is when the curve first appears, the tones are evenly
distributed between black and white as they were shot.

To darken the selected area of the image, click on the curve line somewhere
near the white end and drag the curve down as shown above.

Click 'next' to continue...