Gimme a break. Label chasers and registry addicts regularly pay nosebleed prices for HG CGC books, and raw copies sell for nowhere near the same amounts. That's a fact, and I have never, ever, ever seen a common Byrne X-Men raw copy selling for thousands of dollars, but some CGC copies do.
And using Zilla as an example to support your specious claims is hilarious, as everyone knows his buyers are ingesting pure crack.
GPA measures and reports on CGC sales, so therefore it's only valid for CGC copies. How this can be difficult for some to grasp is beyond me.
But the common Byrne X-Mens you're referring to are likely the 9.8 or highers--which aren't all that common (I haven't checked the census, maybe there's tons, but I doubt it).
As to Zilla, the scary thing is these folks aren't on crack, so some other phenomena is going on.
But I think you're falling for the either-or fallacy. It doesn't have to be either GPA for CGC or not at all; using it as a barometer for raw isn't as invalid as you're making it seem.