In the Fe-I there are 3 axes that pass through the center and are 4 fold rotation axes, so eta=0 and the field gradient tensor will have symmetry.
In the Fe-II two of the axes that pass through the center have 2 fold rotation and the other one has 4 fold rotation, so again since there’s an axis with 2 or more, eta=0 and the field gradient tensor will have symmetry.