The ornamental design for a massagegun, as shown and described.
FIG. 1 is a first perspective view of a massage gun showing our new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left-side view thereof;
FIG. 6 is a right-side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines shown in the drawings are included for the purpose of illustrating portions of the massage gun which form no part of the claimed design.