As far as I know, Google Play Music (the app) stores offline music in it's data directory. This is device dependent, and you'll need a file explorer such as ES File Explorer to access the files directly. If you have the music stored on the device itself (so not a microSD card or external storage), they'll be located in somewhere such as
/storage/sdcard0/Android/data/com.google.android.music/files/music - they are all MP3 files, and the filenames are all numbers, eg:
14.mp3. If they are stored on external storage, again it is system dependent, but one possible location is
Anyway, I'm not sure if these files get deleted when your subscription ends/you cancel it, you you could always copy them to somewhere else (for example,
/storage/emulated/0/Music for internal storage, or
/storage/sdcard1/Music for external storage.