I claim the ornamental design for a guitar hook, as shown and described.
FIG. 1 is a perspective front view of a guitar hook showing my new design in an environment of use;
FIG. 2 is a perspective front view of the guitar hook;
FIG. 3 is a perspective rear view of the guitar hook;
FIG. 4 is a top view of the guitar hook;
FIG. 5 is a bottom view of the guitar hook;
FIG. 6 is a right-side view of the guitar hook;
FIG. 7 is a left-side view of the guitar hook;
FIG. 8 is a rear view of the guitar hook; and,
FIG. 9 is a front view of the guitar hook.
Any broken line illustration of environmental structure in the drawing is not part of the design sought to be patented. (The neck of a guitar is shown in broken lines for illustrative purposes only and forms no part of the claimed design.)