I claim the ornamental design for a gas burner, as shown and described.
FIG. 1 is a top perspective view of a gas burner, showing my new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines are included for the purpose of illustrating the boundaries of the claimed design. The broken lines and surfaces within them form no part of the claimed design.