The ornamental design for gable lighting fixture, as shown and described.
FIG. 1 is a perspective view of a gable lighting fixture showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is an end side elevational view from the left end thereof, the opposite right side end being a mirror image thereof; and,
FIG. 5 is a front side view thereof, the opposite rear side being a mirror image thereof.
The broken lines in the figures represent unclaimed subject matter and form no part of the claimed design. The fixture is shown broken away. The area between the lines that indicated the fixture is broken away form no part of the claimed design.