The ornamental design for a cap of a lip gloss bottle, as shown and described.
FIG. 1 is a perspective view of a cap of a lip gloss bottle, showing my new design;
FIG. 2 is a front elevational view thereof, the rear, left and the right plan views being the same images;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is an elevational view of the cap on a lip gloss bottle, wherein the broken lines are shown for illustrative purposes only and form no part of the claimed design.