The ornamental design for a gas filling apparatus, as shown and described.
FIG. 1 is a front elevational view of the gas filling apparatus of the present invention showing our new design, the rear elevational view being a mirror image of the front elevational view;
FIG. 2 is a right side elevational view thereof, the left side elevational view being a mirror image of the right side elevational view;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a is a top, front perspective view thereof.