This Taiwanese artist is on Spotify but not Google Play Music.
https://open.spotify.com/artist/2JBUyLiFvpFPWdZGqIGYLD?si=uq...
https://play.google.com/music/listen?u=0#/sr/crowd+lu
Nothing except youtube, but that's just cheating.
Same with this one.
Spotify claims to have 35m songs and GPM 40m. Not that that means much, either. The killer feature GPM has over Spotify is that you can upload your own tracks to fill the gaps wherever their library is lacking.
Spotify has the ability to upload music as well, however that is contingent on you having premium.[1]
[0] https://open.spotify.com/artist/3nFkdlSjzX9mRTtwJOzDYB?si=j1...
[1]https://community.spotify.com/t5/Desktop-Windows/Can-you-upl...