To Set the Active Fill to a Gradient

Select a tool in the Polygons toolbox.
From the Fill Type option menu in the Tool Settings window, select Opaque or Outline.
In the Active Color dialog, select the Gradient Fill tab.
From the Type option menu, select the fill type.
From the Color Mode option menu, select the number of colors: 1, 2, or multiple.
Adjust the gradient as desired.