Hi, Steven.
I hope you're enjoying GarageBand so far! It has a lot to offer. As far as I know, the artist of "Curtain Call" has never been released to the public. As far as Apple releasing the full song, the version that comes with GarageBand would be considered the full version of the song. As an added bonus, since it is a multi-track recording, you can edit it, which to me, seems like a 'fuller' version of the song.
If you want the song on your iTunes library on your Mac or PC to sync to your devices, export it from GarageBand as an audio file by tapping the 'Edit' button when you're viewing previews of your songs and when the songs start jiggling, tap on it and tap the 'Share' button (the arrow) and then export it to iTunes, Mail, iMovie, etc.
Hope that helps,
-Erika