I haven't had time to pull the car apart to see what the difference actually is but when I started to answer your question, I went back to RockAuto and I think I have figured it out. It's not that they don't have the right one for the GXP listed under the GXP listing, it's that they have all of the numbers wrongly listed under all of the cars, the GXP, the other LS4 cars, and the regular models (GP, Impala, MC). Since I couldn't remember the difference, I was assuming that they were correct for those applications, which would make them wrong for the GXP. The catalogs just don't specify which ones are GXP specific listings or point out the differences between the numbers. The one pictured above is wrong for the GXP.
I'm pretty sure this one is correct:

RAYBESTOS
712244
Look at the mounting flange, it seems to be a thinner single plate instead of a double plate. It is rectangular where the wrong ones are concave on the short sides, and it's hard to see on this photo, but if you will look at the corners, they are beveled front and back. Since the catalog doesn't list the differences, you just have to match photos to get the correct ones.
Thanks for making me think about this again. No one had responded and it just sort of got pushed to the back burner but when you brought it up fresh, it just sort of clicked.