Fe-I has fourfold rotational symmetry around the axis through the orange atoms or through 2 red ones. For the Fe-IIa atoms there is the same fourfold rotational symmetry as the Fe-I. For Fe-IIb we only do not have a fourfold symmetry. This only has the same symmetry if we lay the rotation axis through both. Fe-IIb atoms.
Eta is zero in both cases and since there are more then two 4-fold rotation axis, the EFG tensor should be zero.