Doc:How Do I

Apply a gradient to an object instead of the entire canvas?

1. Create the region you want to fill with a gradient, and the gradient layer, if you haven't already.
2. Make sure that the gradient layer is above the region layer in the Layers Dialog.
3. Select both layers, right click, and select Encapsulate.
4. Expand the new Inline Canvas layer if it's not already, and select your gradient layer.
5. In the Params Dialog select the Blend Method parameter, and choose Onto from the drop-down menu.

The gradient will clip to the visible area of the region below it inside the Inline Canvas. (and any other layers in that section).