The ornamental design for an extensionpiece for a gutter, as shown and described.
FIG. 1 is a right side perspective view of an extension piece for a gutter showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof; and,
FIG. 5 is a left side perspective view.
The back is plain and unadorned.
The broken lines shown in the drawings depict portions of the extension piece for a gutter and environmental subject matter that forms no part of the claimed design.