What happens to my Google Play Music library when I unsubscribe?
What happens when you cancel your subscription from Google support page says (emphasis added)


When the end of the billing period is over and your subscription is canceled, you won't be able to access music you downloaded to your device, or playlists you created.
If you subscribe again, you'll get access to your radio stations and also the playlists and music you uploaded.


What happens to All Access albums in my library when I unsubscribe from Google All Access? reiterates

If you unsubscribe from Google Play Music All Access and then re-subscribe you will not have the ability to play YouTube videos in the background on Android another setback for some users