That works, thank you very much.
In case the developers are reading this thread, here are my conclusions:
- I was not aware of the "Rasterize to mask" option because it's not in the contextual menu. When I right-click on a layer in the Layers pane I expect all the relevant commands to be available there.
- The method outlined by AffinityFran works but it's a bit convoluted. This is not a criticism to AffinityFran, but to AP. There has to be an easier way of editing a mask. I see a mask as just a bitmap, I should be able to add a monochromatic gradient without going through hoops.
Thank you again for the great help.