Step 1

In a new document, select the Ellipse Tool and draw a 50px radius circle.

[ Register or Signin to view external links. ]

Step 2

With the circle selected, apply a linear fill. Here I use a shade of red and black. Now clone (Ctrl + shift + D ou Edit > Clone) the circle. Now we have two circles, the second circle is now selected and on top of the first one.

[ Register or Signin to view external links. ]

Step 3

To reduce size of the top circle, right-click on the selected circle ans select Transform > Numeric Transform . Choose Scale in the dropdown menu. Make sure that the option Constrain Proportions is selected and give 75%.

[ Register or Signin to view external links. ]

Step 4

You can see that on the selected circle we now have 2 handles, a rectangle handle and a oval handle. And to tweak the linear gradient fill, we are going to rotate the rectangle handle. When your cursor gets close to the rectangle handle, it turns into a rotating icon.

[ Register or Signin to view external links. ]

Step 5

Rotate the rectangle handle to the opposite direction. Press Shift to get a straight line.

[ Register or Signin to view external links. ]

Step 6

With the subselection Tool, now drag the oval handle to the right.

[ Register or Signin to view external links. ]