I'm no expert, but I'm pretty sure Google Music that you "pin" to your phone is stored to the SD card.
When I dismount my SD card and then restart my phone all of the Google Music is gone. I think the reason most people assume Google Music is stored on the internal memory is because it's hard to find on the SD card because of the filing system Google uses.
Hopefully someone with more knowledge can provide clarification.
It is stored on the SD card. Only problem is if you want to play w/another music player, the songs don't show up. When you do find the folder(root needed) all the songs are coded for Google Music to see song info. You have to get an mp3 tag editor that can analyze the song in order to get you your info. This is the one way I found around the 2 d/l max per song from Google Music. It is a PIA!
Only non-root way is just to d/l from Google Music online and transfer to your phone.
Good Luck.