This step is a bonus trick I just learned. First, get your gradient to be relatively skinny. Then, use the corner dot to drag your page size so most of your screen is white. Make sure your secondary color white before you do this. Then, select your gradient using the rectangle select tool, and click transparency. Then, while holding the shift key, drag your gradient in any way you want, such as a wavy line. Lastly, use the fill bucket to fill in the white space around the edges. Voila!