Create a new document and use the Ellipse Tool (L) to create a 280 px by 280 px ellipse.
From the Gradient panel, fill the ellipse with a Linear Gradient with no stroke. Change the first Color Stop in the gradient to a gray color and the second Color Stop to a dark gray color. With the Gradient Tool (G), click at the top of the ellipse and drag towards the bottom, making the gray color at the top of the ellipse.