FIG. 1 is a front side view of a gas cylinder cap. FIG. 2 is a side view of a gas cylinder cap. FIG. 3 is a side view of a gas cylinder cap. FIG. 4 is an isometric view of a gas cylinder cap. FIG. 5 is a side view of a gas cylinder cap. FIG. 6 is an isometric view of a gas cylinder cap; and, FIG. 7 is a top plan view of a gas cylinder cap.