The ornamental design for gun, as shown and described.
1. Gun
1.1: Perspective
1.2: Left
1.3: Right
1.4: Top
1.5: Rear
The break lines have been applied to the drawings in FIGS. 1.1-1.3 and 1.5 to show undefined length. The appearance of any portion of the gun between the break lines forms no part of the claimed design. The broken lines which appear throughout the drawing disclosure form no part of the claimed design.