Single rake is for ground that slopes one way, like a driveway running downhill. The top of the gate stays flat and level, which looks tidy and lines up with the fence. The bottom of the gate follows the ground, so the gap under the gate stays about the same the whole way along.
To measure it, stretch a level line across the opening. Measure from the ground up to that line on the left, then on the right. Because the ground slopes, the two numbers come out different.
In CAD60 you pick Single rake and type the two heights. It tilts the bottom rail and cuts every bar to suit.