The function “ngsmesh.Boundaries(“default”)” gives you a boundary region. Therefore your integrator just integrates over your boundary (where you do not have any volume information) and there the gradient is not defined.
If you set proper boundary conditions, the term is replaced by a function for neumann boundaries and it vanishes for a dirichlet boundary.

If you use a DG method, such terms can make sense. But then you have to use “element_boundary=True” or “skeleleton=True”.
The following integrator integrates over ALL element boundaries (loop over the inner interfaces twices and boundary elements once).

boundaryterm += SymbolicBFI(grad(w) * n * phi, element_boundary=True)

The next one integrates over all boundary elements.

boundaryterm += SymbolicBFI(grad(w) * n * phi, BND, skeleton=True)

It is always easier to help if we know more details of the problem you want to solve.