1. Create the Region you want to fill with a gradient.
2. Create the Gradient Layer. As you can see it fills the Canvas completely.
3. Select all layers, right click, and select Group. Make sure the gradient is above the shapes it should fill.
The gradient will clip to the visible area of the region below it inside the Group Layer. (and any other layers in that section).
You can download the File:Doc Object Gradiants.sifz used in this example.