Gravitational mass is the mass of a body which determines the magnitude of gravitational pull between the body and the Earth This is determined with the help of a beam balance. According to Newton’s law of gravitation, the gravitational force on a body is proportional to its mass. We can measure the mass of a body by measuring the gravitational force exerted on it by a massive body like Earth.
If FA and FB are the gravitational forces of attraction on the two bodies of masses mA and mB due to the Earth, then;
FA = GmAM/R2 and FB = GmBM/R2
where M is mass of the Earth. R is the radius of the Earth and G is the gravitational constant.
So, mA/ mB = FA/FB
If one of the two masses is a standard kilogram, the unknown mass can be determined by comparing the gravitational force.