The ornamental design for a cap for a motorcycle gas tank, substantially as shown and described.
FIG. 1 is a top perspective view;
FIG. 2 is a front elevational view, with the rear elevational view, the left side elevational view and the right side elevational view being mirror views thereof;
FIG. 3 is a top plan view; and,
FIG. 4 is a bottom plan view of my cap for a motorcycle gas tank.