Gaps which are closed at one end and open at other end sometimes get failed to fix using Gap tool. Even the Fill tool may not work. The surface bodies forming the gap may have face type as spline which sometimes cause difficulty in removing such gaps. One way to fix this issue is to first widen the gap so that there is opening on both ends. To increase gap distance, select any edge on one side of the gap and Pull or Move it to widen the gap. And then use tools like Blend to create a new surface between the gap.