I claim the ornamental design for a guitar backpack, as shown and described.
FIG. 1 is a top perspective view of a guitar backpack, with the lid shown in an open position, showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a top perspective view with the lid shown in a closed position;
FIG. 4 is a bottom perspective view of FIG. 3;
FIG. 5 is a left end elevational view of FIG. FIG. 4;
FIG. 6 is a right end elevational view of FIG. 3;
FIG. 7 is a left side elevational view of FIG. 3;
FIG. 8 is a right side elevational view of FIG. 3;
FIG. 9 is a plan view of FIG. 3; and,
FIG. 10 is a plan view of FIG. 4. ps The broken lines showing a guitar in FIG. 1 depict environmental matter and form no part of the claimed design. The remaining broken lines represent stitching, and are part of the claimed design.