If you want the video and audio to automatically sync, Final Cut Pro X is the tool of choice, but it is more expensive than iMovie, and it has a big learning curve.
With iMovie, you can still do this by showing the wave forms and lining them up visually. There is a nuance that in iMovie, you must have video. You cannot have sound that exists outside a video element. For you, this means that if your GB track is the same length or longer than your iMovie video track, you cannot shift it side to side. This is easily fixed by adding some dummy clips before and after your video so there is room to slide the track back and forth. You can find these "Black Background" clips (and others) under the Maps tab.
I recently had to sync up some audio with video. Here is a post that explains in detail how I did it. It may help. Unfortunately, in my case, I was syncing the audio from 2 cameras. This is easier than syncing video with an audio track, but some of the tricks may still apply.
If anything still does not make sense, please ask.
One more thing. When you have an audio track like one created in GB, it is not simple in iMovie to delete a chunk of it and delete the corresponding video at the same time. Once again, this is a task better suited for Final Cut Pro X. It is possible to export your movie to the Media Browser and then continue editing it in GB, so that may be an option for you.
I have Imovie '08 and by what I have been reading, it's very hard to sync audio and video.
WHat I did was record myself singing with a video camera and concurrently record the song in Garageband. I can't remember how I did it a year ago, but all I did was sync the sound from the camera and the one from GB, but this time, it seems as though no matter what I try, it's never right on. It always ends up a little bit off. I tried using "trim", but all that did was cut some pieces of the song. I also tried matching the waveforms but I can't get them large or spread out enough to sync exactly. Is there a way to this?
As I said, I can't remember the steps I took about a year ago, when I would do it very easily.