The ornamental design for a gem pavilion, as shown and described.
FIG. 1 is a bottom plan view of a gem pavilion 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 front left corner elevational view thereof, the rear right corner elevational view being identical to the front left corner elevational view and the rear left and front right corner elevational views being mirror images of the front 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.