The gun was developed for the plane, not the other way around. The original prototypes flew with the M-61A1 Vulcan because the Avenger cannon was not ready yet.
I posted some pix i took of a GAU-8/A with the service bays open at NAS Willowgrove when i was visiting a stick actuator that posts at my board up there(it's about 25 minutes from my house). Good close up detailed stuff.
Here's a link:
GAU-8A Avenger pix
Some buddies of mine over in Iraq also took the liberty of personalizing some 2.75" rockets for the army guy that runs by far the best A-10 site on the planet(Yes those rockets were fired in anger):
www.a-10.org