I don't know if you have tried this but, what if you first mute the garage band audio and find a decisive sound on your camera footage - a drum beat at the end of a song, for example. Place a marker at that point (press M).
Mute your camera audio and switch the garage band audio on. Find that same sound and place another marker.
Go to "Mark" in the FCP menu bar and choose "Mark to Markers" - sorry I forget the shortcut, FCP is busy capturing so I can't look. This will show you how far apart the sounds are and you can slip the garage band track into position.
When you made your recordings, there was no direct relationship between the two, apart from being in the same building. So pressing record on the camera and starting garage bands record function did not happen at the exact same time, This probably accounts for the discrepancy.