Grade of service and quality of service is something related to each other.I mean if you improve one , so this means u improved another. If you become able to get less grade of service so it means that you improved quality of service. This means that the system will be less conjested by having good quality of service.Thus grade of service and quality of service is some thing related to each other.
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You are partially right, but notice that:
GoS: means the (blocking or delaying) probability as your Erlang formula.
QoS: depend on three issues, (Integrity, Retainability & Serviceability)
Thus the relation between two just through the serviceability)