That's not possible (not yet anyway) in Draw. But... I'll pass the suggestion along to the product team.
Sorry to bring up and old thread. I just started using Draw with Apple Pencil and gradient fills for backgrounds would be great. Draw is an amazing app with so much potential.
Although I know it's discussed, I don't know what the plan is for adding this functionality so I just continue to share the requests with the product team.
I'm sorry I don't have more information (even after such a long time).
I'll +1 on that request. Just to let you know there is a demand for it on this side.
Oh and please don't forget about us Android Samsung Note 10.1 users! Still a brilliant device for the job, even though the Android version is lagging behind the iOS one.