The little ball-bearing assembly in the rotor might do with a bit of lube...
That is, unless you really don't have the screw tight enough and the rotor is rubbing on something.
All of the Noob Subs take an odd sized crystal so I doubt the GMT would be any different. You could pop it out, measure the diameter and thickness, and go from there. I seem to remember the Noob crystal is made a bit differently than others so you might have to bed in a replacement with clear epoxy.
Post a picture of what you are doing, I think something is getting lost in the translation. It sounds like you are wanting to remove the lume from a metal "surround" and that would account for there not being a "hole" as the metal pearl's shell is a cup. Very interesting if you were able to glue in a vintage pearl "dot" into the cup! Again, pics are needed...
I don't have any of these crystals. I had a need for a crystal and after I came up with this as a possible replacement I ordered an extra and sent it to Mr. Goat for him to play around with.
You can order this crystal from Jules Borel under part number MG25 295.