GExtrude quickly creates a watertight modeling domain from a given surface mesh (e.g., a topography, meshed surface scans, etc.) This is done by extruding a surface mesh to a bounding surface with any orientation. There are options to create structured or unstructured mesh for the extruded parts and to keep the extruded parts separate or merged with the original mesh.
Note that the new mesh (domain mesh) is not automatically intersected with any other meshes (except for the initial extruded mesh) and, therefore mesh intersection (GInt) and remeshing operations (GSurf) may be needed.