GB isn't really designed as a tool for breaking up audio files in the way you want to. I mean it can be done easily enough using the method I described but it's not the sort of thing I'd expect to find as an automated process in GB.
You might be better off working with a CD mastering program that is specifically used for preparing tracks for an album on CD or whatever format. Those programs come with an interface and tools specifically designed for setting up all of your tracks' in and out times, overlaps, mastering, and whatnot.
Usually one brings in all the already-separate audio files to work with but I'd think a program such as that would also work well for your needs.
Here's a program that a quick google search for "automatic audio cutting" got me.
http://direct-wav-mp3-splitter.pistonsoft.qarchive.org/
It looks like it could do what you want. And it looked like there were a number of other apps out there like it. Not actually sure if that one works on Mac, but I'm sure there's something like it at least on the Mac.
http://www.petergreenstone.com