Do you mean G/Gmax (just tried to autocorrect it to mail for me so I take it you are!)
f-d is right.
That graph shows that the more strain you have the less stiffness you get.
Retaining walls are typically small strain problems and they benefit from using small strain stiffness. For example in plaxis you can use the hardening small strain model (HS small). You select the strain rate, for example 0.001. As your strain increases it uses your shear modulus (0.7 x Gmax) to a strain of 0.001 to determine deflections. After that it uses your standards Young’s modulus to determine deflections.