Double rake is for jobs where the top of the gate has to slope as well as the bottom. This happens when the fence on each side sits at a different height and runs at its own angle. The top follows the fence and the bottom follows the ground, each at its own slope.
You measure four things, all from the same flat level line. At the left post, measure up to where you want the top, and down to where you want the bottom. Then do the same on the right. Four numbers.
In CAD60 you pick Double rake and type those four numbers. It tilts the top rail and the bottom rail on their own angles and works out every cut.