By default, outlines are filled with solid colors, however if you need to apply a gradient fill to them, follow these steps:

1. Select object and go to: Object > Convert outline to object – or use the shortcut: [Ctrl + Shift + Q]
    This action transforms the outline to a new object (*)
2. Select the new object and press [F11] to perform a gradient fill and edit colors as you wish.
(*) This is also useful if you need to resize object and keep “outline” with proportional thickness.

Tip provided by Silvio Gomes, CorelDRAW Master, Artist and Graphic Designer.