Use these instructions to allow your offline Google Music files to be accessible by 3rd-party music players. You'll need to be root-access enabled before continuing.
Prerequisites (get these before you start):
The first thing to do is figure out where Google Music is currently storing its files. This location will differ between every device and manufacturer, so figure out where yours are and use this information throughout the rest of this tutorial. I'm using a Samsung Galaxy S3, so for me Google Music stores its data at the following location:
Figure out where this folder is. For you, it may be in /data/com.google.android.music/ directly in the roof of your internal memory. Look around until you find it.
Inside this folder, you should find a few subfolders, but there are three you're going to concern yourself with. These are:
The first thing to do is delete these three folders. WARNING: From this point on, you will lose any offline music you already have saved. If you don't erase them now, they will continue to take up space on your internal memory but become inaccessible. Don't worry, we'll be recreating them soon enough.
Go to the root of your external SD card (for me, this is /sdcard/) and create a directory called MusicCache. Inside of this folder, create three more folders, cache, files, and databases. Make sure to spell them exactly like this, all lowercase.
Next, open up Android Terminal Emulator, and you're going to type in four commands, following each one with a press of the Enter key. Remember, you'll need to change the paths for each command to reflect where the files actually are on your device.
ln -s /data/data/com.google.android.music/cache/ /sdcard/MusicCache/cache/
ln -s /data/data/com.google.android.music/files/ /sdcard/MusicCache/files/
ln -s /data/data/com.google.android.music/databases/ /sdcard/MusicCache/databases/
(After you type in the first command, you'll need to grant Terminal Emulator root access using whichever Superuser application you have installed.)
If you've made it this far, you're nearly there. Everything from here on is done within Google Music Exporter itself. If you are on a newer Samsung device, you've likely been able to follow this guide to the letter and have everything installed to the same directories as I've used. If this is the case, you're now good to go, just open up the app and Export All Offline Music Now.
If you're on a different device, you'll need to change the paths under Advanced to reflect where the files are actually stored on your device. Just click on them and change the values, then you'll also be good to go. Export All Offline Music Now, and you should be golden.
Export All Offline Music Now will run a full metadata transfer, whether your files already have metadata or not. Use this the first time you run Google Music Exporter, or if you need to reset the metadata to whatever it originally was.
Export Only New Music will skip over any mp3 files that contain a valid Title tag (basically anything that has already been processed by Google Music Exporter). You should use this setting most frequently.
Contact me at firstname.lastname@example.org. Just put "Google Music Exporter Help" as the subject. I'll reply quickly :)