Step 1
Fill your canvas with the color you want at the top.
Step 2
Create a triangle for the bottom color. (It can face either direction, just make sure it is not flipped vertically, unless you want the wedge color to be the top color.)
Step 3
Adjust the size so that the image is one pixel wide. (Make sure that pixels is selected rather than percentage, and that the "Maintain aspect ratio" checkbox is deselected at all times.)
Step 4
Adjust the width, and voilà! You have an instant gradient overlay! You can also use this handy tip to make a horizontal gradient, just resize vertically instead of horizontally.