If you download a song from the Google Play Music app to your device, it's actually difficult to access, because the file has a number code for a filename, and the directory is pretty obscure. Consider it a soft way of discouraging rampant piracy. Anyway, the best way for you to accomplish what you want is probably to do the following:
1. On your computer, download the music you want from your Google Music account. You can do this either with the Music Manager, or directly from the Google Music website (the one that shows your whole library). Using the Music Manager, you can download your music an unlimited number of times, but the catch is that you have to download your ENTIRE library, which can be a pain if it's huge. If you download from the website, you're limited to downloading a given track a total of twice--so if you download here, keep the file somewhere safe.
2. Once you have your music file downloaded, then transfer it to your Nexus 7 using AirDroid, and then add it to your RocketPlayer playlist.