I'm not sure about ALL Gold Cups, but I know that some Gold Cups (and maybe all?) have been made with lightened slides (metal cut out in some areas) to facilitate them shooting "match" ammunition more reliably.
I've been told they run the risk of fracturing using full power ammo.
As I said, I don't know if this holds true to all versions of the Gold Cup or not, but one that I had was this way.