This app was mentioned in 12 comments, with an average of 4.00 upvotes
I don't wanna be a dick or anything but you can build alternative clients.
I know because I have. I built a little client that let multiple users control the same playlist, as an alternative to Sonos for my office.
It got like 70% done and we just sorta... Stopped.
I'm not sure why people haven't done it but it's certainly not because it isn't possible.
edit: I had a quick look and on Android there's: https://play.google.com/store/apps/details?id=org.tomahawk.tomahawk_android
Which also supports Spotify. So they do exist. Perhaps it really is that most of the people doing it are doing them as hobby projects and they never get released.
Also, it can get pretty complicated when you consider all the little things you gotta keep track of and be updating constantly.
I personally have never tried it. But you may want to check out Tomahawk, not sure if it is going to do exactly what you want but from what I remember it was marketed as a personal/p2p Spotify.
https://www.tomahawk-player.org/
https://play.google.com/store/apps/details?id=org.tomahawk.tomahawk_android&hl=en
There's Tomahawk. It also supports Spotify, SoundCloud, and several other streaming services, but it's still in beta and it shows. There's a desktop client, too, if that sort of thing tickles your fancy.
Tomahawk is the only one I can think of but it's still very WIP. I do really like the idea of having all your music streaming platforms in one place, though.
Tomahawk has a plugin to stream from GPM, as well as Spotify Premium, Soundcloud and many other services. The app is still in beta and might have some bugs.
The App: https://play.google.com/store/apps/details?id=org.tomahawk.tomahawk_android
For more info about all of the plugins and the Desktop apps, you can go here: https://www.tomahawk-player.org/
No, you'd basically have to make a 3rd-party app.
Speaking of 3rd party apps, I've heard Tomahawk with its Spotify plugin and Fidelify are pretty good.
(Currently beta)
Tomahawk Player. I haven't tried it myself though.
https://play.google.com/store/apps/details?id=org.tomahawk.tomahawk_android this is pretty interesting