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Inertial mass is found by applying a known net force to an unknown mass, measuring the resulting acceleration, and applying Newton's Second Law, m = F/a. This gives an accurate value for mass limited only by the accuracy of the measurements.There is no difference between gravitational and inertial mass as far as we know.One method to measure an unknown force is described here.A force stretches the spring - higher the force, longer the spring. A certain spring extension means a certain weight and a certain weight means a certain force.Say the unknown gravity force applied on a weight of 1 kg stretches the spring a certain length. The same unknown force is applied by gravity on any other object that stretches the spring the same length as the 1 kg weight does. To quantify this force we have to define or invent units arbitrarily in a convenient way. In order to do this, the force applied by gravity on a mass of 1 kg is defined as 1 kilogram-force. So 1 kilogram-force is our basic force unit.