Same way you would use a synthesizer.
I thought Al elliot was talking about an Audio Unit Effect, something that you select as an Audio unit that could take an Aiff file or a WAV file and manipulate it as you might with a real turntable in a manner you wish, sometime during the course of your song. Much the way you would use a synthesizer in a manner you like during the course of a song.
I have an AU called Turntablist 1.1 that almost does the trick but I can't get it to work in a defining manner with GB. The site states the DL as a VST but I believe the package contains the AU that I have also.
Here is the description:
http://www.macmusic.org/software/view.php/lang/en/id/2544/
scratching
Turntablist is a record scratching emulation instrument that was originally developed by bioroid media development.
Another AU that almost gets the job done is Musolomo.
http://www.macmusic.org/software/view.php/lang/en/id/3125/
I can't find a way to manage the controls in GB.
Now if you could find an AU that could load a sample and allow you to use that sample as an instrument the way a turntabilist scratches with a vinyl turntable you could use it in your song. Then you wouldn't have to use already made scratch loops or soundfonts and you could control your own sound.