He probably wants “Gradient theme color” and/or “Gradient dynamic color” as a background option for compact now playing. Currently there is only “Dynamic background”, which is no gradient.
I agree though that it would look strange, because the color would change rapidly on a very short length in the compact NP, and also this would create issues with the button colors. E.g. if the top color is dark and the bottom color is light, it is hard to decide if the buttons should be dark or light to be fully visible.