The ornamental design for a gazebo, as shown and described.
FIG. 1 is a side view of a gazebo showing my new design with round posts having extensions projecting past the circular support and with transverse stays extending from the round posts to the circular support, wherein a view from the opposite side of the gazebo is identical.
FIG. 2 is another side view of the gazebo rotated by 90 degrees relative to FIG. 1, the view from the opposite side of the gazebo being identical.
FIG. 3 is a top perspective view of the gazebo; and,
FIG. 4 is a top plan view of the gazebo.
Broken line areas in the illustrations depict portions of the gazebo that form no part of the claimed design.