Click the New Fill or Adjustment Layer
button at
the top of the Layers palette, and choose the fill type you want
to create. (The first three options listed in the palette are fill
layers, the others are adjustment layers.)
Choose Layer > New Fill Layer > [fill type]. In the New Layer dialog box that appears, click OK.
You can also specify the shape of the gradient (Style) and the angle at which it is applied (Angle). Select Reverse to flip its orientation, Dither to reduce banding, and Align With Layer to use the layer’s bounding box to calculate the gradient fill.