Fran, perhaps you're refering to the "create new fill or adjustment layer" button at the bottom of the Layers panel whose icon is a little circle half black and half white. Select the layer you want to make the change on and click that button and select "Gradient..." a small window will pop up and there you can select the style type of gradient you want rotate, scale and reverse it. By LMB clicking the gradient box there you can also edit the gradient colors, placement and opacity.