I believe the Gray Market guns refer to basic MX-8's, Mirages, etc, that have been upgraded by adding Perazzi factory engraving patterns, sometimes by the very engravers that Perazzi contracts with for the work.
The result is a gun of equal or better quality engraving that looks just like an SCO or SCO Gold, but has been upgraded to that level independent of Perazzi. The result is the same gun, same engraving and cost savings tot he owner of $5,000 to $10,000. I have one of these guns.
It's my understanding that Perazzi continues to actively discourage this activity anong the dealers and engravers.
The Hack