I was using Nova on the Sony Z3, and the widget for the Walkman Music app. was there and worked fine.
I don't like Google Music, so switched to Rocket Player when I got he G4, and no support for the widget on Nova.
As it goes, I'm perfectly happy with the stock launcher experience on the G4, so switched back, but Nova does have some limitations, widget-wise, for all it's malleable bells & whistles.