How to check if wall surfaces have gaps?

I have created lod2 buodings by using the transformer BRepSolidBoundaryCreator.Beta. After this I used SurfaceonSurfaceOverlayer and tester (overlaps < 2) to remove internal walls. Afterwards I am using GeometryValidator to check if there are some errors inside. I have some Self Intersections:

When I use "Self intersections in 2D" how can I check if the repaired geometries are without gaps?

Could it be that during the repair some fixed polygons have gaps to the neighboring polygons?