You can add a Gmask Plane node to the schematic in order to accomplish specific tasks such as creating wipe effects or having better controls over reflection effects.
A GMask object (called gmask1, by default), with its parent axis, appears in the schematic, and a single straight spline that is opaque on one side and transparent on the other, appears in the image. You can use the manipulator handles to adjust the Gmask Plane, and add as many gradient points to the spline as you want to.