Pattern gradient maps controls fades and gradients. From left to right, the map corresponds to the Green (color lookup) channel of the Pattern. Top to bottom controls the angle of view, so you can, for example, create a Chrome Effect. Also shares the same Angle color change functionality as the base gradient.
The Pattern Gradient should always be 256×256, and can not be changed with 72 ppi.
Save the file in RGB mode as a TarGA file (*.tga) with no compression and with Alpha Channels enabled in 32 bits/pixel resolution.