The ornamental design for a gunstock, as shown and described.
FIG. 1 is a front perspective view of gunstock showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left end view thereof;
FIG. 5 is a right end view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a rear perspective view thereof;
FIG. 9 is a front perspective view showing another angle of my new design; and,
FIG. 10 is a referential operational and referential front side view of the gunstock with a gun body, wherein the broken lines in the drawing is for illustrative purpose only and shows the portion that forms no part of the design to be claimed.