You can add custom music in the Spotify desktop client, and if you have a mobile device on the same network, it'll automatically upload it to that device as well. Not cloud based though.

Spotify does let you upload your own music. (I’m a paying customer though so you might need that.)

Overall I’m very happy with Spotify but experience has shown that within a few years it’ll begin the slow and unstoppable turn towards Villainy.

Just tried it. My experience might be abnormal, but it seems glitchy and slow. A playlist that I already created with my offline songs synced with my phone, but playlist said 0 songs. No error messages, very unintuitive. Had to create a new playlist and add the songs again, still wasn't showing. Relaunched spotify on both devices two times and it finally showed up. And this is for ~20 songs. I wouldn't want to use this method on a big library.

Also, Spotify doesn't upload your music for streaming, its just a sync between your phone and computer that have to be on the same wifi, and keeps the songs downloaded on your phone. Wouldn't work for a collection bigger than your phones storage. I wouldn't even compare this to GPM.

Exactly. I can't play my 'uploaded' music on Spotify from the app on my TV

Apple Music allows this, and might be a better solution for you. A client is available on Android and Windows as well.

I'll second Apple Music. Transitioned to it from GPM".

Interesting to find out the Apple music has an app on Android. They actually might be the best alternative for me I've found so far

I've added a few of my own albums to Spotify. It's a tad convoluted but not difficult.