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 right, rear, and left elevational views being identical to the front elevational view; and,
FIG. 3 is a front right corner elevational view thereof, the rear right, rear left, and front left corner elevational views being identical to the front right corner elevational view.
It being understood that the broken lines are for exemplary illustrative purposes only and form no part of the claimed design.