The ornamental design for a gem crown, as shown and described.
FIG. 1 is a top plan view of a gem crown showing my new design;
FIG. 2 is a front elevational view thereof, the rear elevational view being identical to the front elevational view;
FIG. 3 is a right side elevational view thereof, the left side elevational view being identical to the right side elevational view; and,
FIG. 4 is a rear left corner elevational view thereof, the front right corner elevational view being identical to the rear left corner elevational view and the rear right and front left corner elevational views being mirror images of the rear left corner elevational view.
It being understood that the broken lines are for exemplary illustrative purposes only and form no part of the claimed design.