The rattle is not the ejector...
All GP100s have a transfer bar that prevents discharge of the rounds unless the trigger is pulled all the way back. The transfer bar rattles on all GP100s. If you pull the hammer back all of the way, with an empty cylinder, you'll see the transfer bar flappin' around.
I've had two double action Ruger revolvers, and a single action one. The transfer bars rattle on all of them. I'd be worried if it didn't rattle.
The ejector rod as mentioned earlier in the thread ejects empty cases, nothing more. It will not unscrew, and it doesn't serve to secure the front of the cylinder to the frame like revolvers from other manufacturers.