Based on the GM part number, the front and rear calipers are GXP ONLY, but I assume that this is because of the red powdercoating from the factory.

I don't think that GM would have designed a caliper that was only used on the '04-'05 Bonni GXP. I would guess that the same calipers (in natural finish) were used on a number of other applications.

Does anybody have any knowledge of the other GM vehicles using the same caliper design?

cadillac seville, deville

They are different, use bigger rotors than an SSEi, along with a nicer suspension

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.