This is not a feature I was really looking for in a smart watch, but as of right now I have the iHeartRadio app on my Gear 2 Neo. I had never used it before writing this answer. It does not play on the watch, it only allows you to play or pause the music on your phone. It will also allow you to select a station from your favorites.
There is also a media controller app, which does the same thing, just with whatever media is currently open on your phone.
I've been sitting here playing with it for about 10 minutes and do see a way to make the music play on the watch. Even if there is a way, I'm not sure how it would work in the situation you are describing because the Bluetooth range is rather limited. Or how it would work if your phone is off since the watch has limited storage capacity.
TL;DR: I do not think it will do what you want.