I claim the ornamental design for a gun sleeve, as shown and described.
FIG. 1 is a front right perspective view of a gun sleeve showing my new design;
FIG. 2 is a rear left perspective view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a front elevational view thereof;
FIG. 8 is a rear elevational view thereof; and,
FIG. 9 is a left side elevational view thereof with a gun shown in broken lines. The broken lines are for illustrative purposes only and form no part of the claimed design.