The manual for P2020 (yes, it's error-ridden) says that a selection can be filled with a gradient, by dragging within the selection. Not true. The whole canvas is filled.
Make a selection on a blank canvas with oval tool (I'm still learning what Painter 2020 can do). Choose gradient in toolbar. Drag within selection. Gradient populates whole canvas.
Well well, I restarted Painter2020 and now it works as it is supposed to.
Ooo, glad it's working again!