The gap
Deleting faces or running a boolean leaves holes and n-gons. Blender's Grid Fill (tested on 5.2.1 and 5.2.2) fills rectangular holes with matching sides cleanly, and it also fills uneven borders with an even edge count. On those uneven borders it either moves the grid corners off the real corners, which leaves thin, skewed quads, or adds triangles when you select only two opposite sides. On concave L- and U-shaped holes its default result has folded faces and vertices outside the hole. QuadMend keeps the real corners, joins uneven sides with transition rows and splits concave borders before filling: every even border it filled in our tests came out as quads only, with no folded faces, and an odd hole or odd n-gon fills straight from the selection with one triangle, where Grid Fill created nothing from the same selection.
