From LedHed's Wiki
Google Play Music hosts your music in the cloud and you can stream or sync that music to your mobile device.
Music (Cache) on SD Card
By default Google Play Music caches music to your internal storage.
If you want to move the cache to an external SD card then you need to create a symbolic link to your SD Card.
Create the following folder on your SD Card:
mkdir -p /storage/sdcard1/gMusic/music
Create a symlink to it in the following folder:
cd /data/data/com.google.android.music/files ln -s /storage/sdcard1/gMusic/music