First we set up a gradient.

If the patches at the top are not set to black and white, click on them and set them.

Next click on the triangle on the patch below "Styles" and select Gradient.

A gradient will show up. Click in the middle of the patch and the requester far right will pop up.

If #1 Foreground-Background is not selected, click on the little triangle beside the gradient window and set it. Use settings similar to here with the angle matching the direction of the selection.