Brownells does not list an MMC sight for Gold Cup. I do not know of a replacement sight that does not require machining other than the Millett, but I don't see how it would be any better, the problem is not the sight, but its attachment. There is just nothing there to mount to except the small pin hole and elevation screw hole.
I have read that the old Colt Accro sight is somewhat more durable, the sight blade is recessed but it fits the same cut and is held down no better. I have also read that you can help the Eliason or Accro by replacing the roll pin with solid - something like the shank of a good quality drill bit, or a piece of spring wire, or welding wire. Size is 1/16".
And also by putting a small o-ring around the elevation screw or just a scrap of rubber wedged under the sight leaf to cushion it against recoil.
If you want something nice, Wilson makes a sight to fit, but it requires machining in for secure attachment. It does cover the Colt cut, which a Bomar would not. And they will install on a two-week turnaround. Not cheap, though, $215 for their regular sight, probably about the same for GC. I don't know if that would require a new front sight, but they would surely tell you if it did. Call 'em up.