Q: Use GarageBand Music In my Game.

If you made it with GarageBand's instruments, loops or live recordings of yourself playing or singing, and you haven't copied somebody else's copyrighted lyrics or tune, then you should be all good. See Using royalty-free loops in GarageBand with commercial work - Apple Support. You can probably get a better quality file if you export to iTunes and then copy off using the iTunes File Sharing area.

tt2