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.
Hi Petrarch 101,
Hmm, maybe you can tell us which steps you are doing and maybe we can help you get the result you need?
Gradients can definitelly be placed within selections, it's MEGA powerful, see attached images..