It's pretty much all drag and drop. Drag the song from iTunes into the GB window. Trim to the part you want. (I've actually done several longer than 30 seconds no problem) Then under the Share menu, Export to Disk. (save to desktop or somewhere) Then change the extension to m4r, then drag into the iTunes Ringtone window... done.
About the only weird thing with GB is that the timeline is done in BPM instead of minutes and seconds.