GM meaning in Measurement Unit ?

The gram (alternative spelling: gramme; SI unit symbol: g) is a metric system unit of mass.

Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre [1 cm3], and at the temperature of melting ice" (later at 4 °C, the temperature of maximum density of water). However, in a reversal of reference and defined units, a gram is now defined as one thousandth of the SI base unit, the kilogram, or 1×10−3 kg, which itself is defined by the International Bureau of Weights and Measures, not in terms of grams, but by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 kg⋅m2⋅s−1.