how to rectify gap in surface in solidworks?

Those "gaps" are not gaps, they are defects.
One is caused by two surfaces overlapping. The other is caused by the surfaces not even being close to each other.
Gaps are tiny. These issues are quite large.

How to fix? These steps go along with the attached image.
1. Draw a sketch to encompass the defect.
2. Use a split curve to divide the model faces along the sketch from step 1.
3. Delete the problem model faces.
4. "Close" the open edge of the model. Here I used a 3D sketch with a two point spline. The spline is coincident and tangent to the ends of the model.
5. Make use of a tool like Boundary, Loft, or Fill to "rebuild" the missing faces. I used Fill. It is tangent to all model edges, and has a contact constraint with the spline used in step 4.
(6). It was not needed in this case since the fill tool will merge model faces, but if another tool were used, the new model faces would have to be knit back to the main surface.

If the defects did not run all the way to the edge of the model, it may be a good idea to try and repair with the Untrim tool. Even the Delete Face tool could try and fill, or repair the model. But with on open edge, I think the spline sketch will give better results.

Step file of repaired surface is attached.