The ornamental design for a gun, as shown and described.
1. Gun
2. Gun
3. Gun
Designs 1, 2 and 3 are for a gun; 1.1, 2.1and 3.1are front left perspective views of a gun; 1.2, 2.2and 3.2are front right perspective views thereof; 1.3, 2.3and 3.3are front views thereof; 1.4, 2.4and 3.4are rear views thereof; 1.5, 2.5and 3.5are left views thereof; 1.6, 2.6and 3.6are right views thereof; 1.7, 2.7and 3.7are top views thereof; 1.8, 2.8and 3.8are bottom views thereof; the broken lines depict portions of the gun that form no part of the claimed design.