The ornamental design for a gas fired heater, as shown and described.
FIG. 1 is a front perspective view of the gas fired heater;
FIG. 2 is a further front perspective view thereof;
FIG. 3 is a right side perspective view thereof; and,
FIG. 4 is a left side perspective view thereof.
The bottom of the design disclosed in FIG. 6 is included for the purpose of completeness, but no claim is made to the ornamental features to the extent that they are shown only in FIG. 6.