what causing my 2006 g6 to shimmy terribly when I brake, already changed rotors.

Did you replace them using factory pads and rotors? It's probably not the pads causing your problem but the rotors. It's said that rotors should be laid flat when stored. A lot of auto parts stores don't do this because they have so much inventory. Another cause could be failing steeering or suspension part/parts. Bad Tie rods cause this most often but you can't exclude all the other parts until you have it looked at by a mechanic. Also if your tires are out of balance it would be more pronounced when breaking. You will need to have the car checked out as to what the problem is since there are so many thing it could be. If you know how to check the front end do so.

You DO NOT have to use Factory Pads & Rotors. The problem is called "Lateral Runout" GM has a tech Bulletin 00-05-22-002L. You are allowed .002 of an Inch "Runout" anything exceeding this causes a pulse in the braking system. You must either replace the wheel bearings and rotors at the same time (most expensive) or have your rotors turned on the car with a On The Car Lathe (Cheap and Best way). GM uses a plastic shim that fixes runout with a .003, .006, .009 shim. Use a dial indicator and find your top dead center on the rotor and place the shim 180 degrees away from top center(also an option if you have the tools). Buy OE Equivalent Parts (not cheap remember you get what you pay for) and do the job right and you should have no issues. P.S. A lot of stores DO NOT store on the side.