It's not exceptionally well known that the GX470 is a fancier version of the Toyota Prado 120 Series Land Cruiser. Most of the body, frame, drivetrain, etc. will interchange between the two.
Putting an ARB meant for the Prado on the front of the GX isn't unheard of. It requires some... trimming... to fit properly. What kind of trimming? Well, fender trimming. The ARB instructions aren't terribly difficult to follow and will suffice for the majority of the install.
Another point the ARB directions leave out is the removal of this bracing. I've highlighted it in orange.
With some minor fitment and a total lack of concern for cutting a pristine bodied luxury vehicle, the bumper fit!
I spent a good portion of time tweaking the fitment and trimming small portions off the fenders until I felt positive it fit perfectly. Make sure you are 1000% happy with the fitment of the bumper and go through the 'pinning' process listed in the ARB instructions. (ARB recommends leaving around 3/4-1" of air gap between body and bumper)