I happen to know the (former) Delphi DR engineer for the GXP brakes/suspension. I asked him. He recalled that the "natural" version came from the Cadillac SRX. I checked on Ebay and found the attached picture.
According to the listing, the naturals are found on:
CAD SRX 04-08 FRONT RH
CAD STS 05 FRONT (ABS), opt JE5, RH
CAD STS 06-08 FRONT base, opt JE5, RH
He couldn't remember where the rotors came from for certain, but recalled that F-body rotors could be used if the bolt holes were drilled-out for the slightly different hole pattern. He also thought the SRX rotors were the same but had a 6 bolt pattern? I don't know... his memory.... it has been several years.
Now, for Gosh sakes, this is all speculation, until someone can verify.