The ornamental design for a structural gapfiller, as shown and described.
FIG. 1 is a front perspective view of the structural gap filler;
FIG. 2 is a right side view thereof, the left side view being a mirror image thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a front view thereof; and,
FIG. 6 is a rear view thereof.
The structural gap filler is shown with symbolic breaks in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.