GD Square of a motor

I want to know about what is the GD square ( Gyrational Torque) of the motor. I am aware that higher kW motors for e.g 5MW motors load
torque curves are checked by overlapping with the load curves i.e, for e.g pumps to check weather the selected motor will able to start the load or not. in this scenario i want to know very particularly about the meaning of GD square of the motor and a Pump and also the Wk^2 (Moment of inertia J ) of the motor / load ----- Finally how i have to cross check the values with motor and with load.
How those values are calculated - can anyone suggest a formula to arrive those values for a motor please.
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