You're correct; the 288 was never officially designated as anything other than GTO. However, for the sake of clarity in conversation, I find that it's often necessary to include the admittedly unofficial "288" designation just so the listener knows exactly which GTO I'm talking about. Naturally, I never need to clarify "Ferrari" or "Pontiac."
It's my understanding that only some VIN plates have the 288 reference, not all. I wish I knew why it's on there. It is officially called the GTO. The only place on the car that says "288" is the VIN plate on the steering column. The owner's manual, the rear of the car, and every where else, the name is simply GTO. The 288 reference came about because of the car's 2.8 liter V8 engine.
All production cars I have ever seen have "288 GTO" etched into the steering column VIN plate... but I believe you are correct: the official designation by the works is simply: "GTO"
Just rolls off the tongue, doesn't it? And it's fun to say: "Efe quaranta, efe cuaranta!" By the powers vested in me, I hereby decree that this shall be the way we refer to this car from this day forward! And it must be spelled this way too!
Okay, in case anyone cares, I got the information to clear up all of this GTO stuff. When the car was still being finalized, the name "288" came about (2.8 liter, 8 cylinder), to go in line with the 308, 328, etc. However the name GTO (minus the 288) was considered more in line with the specific product, and that's the name that was used on all of the documentation that was distributed to the press. And so, the name "GTO" was, always has been, is, and always will be the official, full name of the car. I hope that clears things up for everyone (who cares).