Gram Atomic Mass (GAM):

The atomic mass of an element express in gram is called gram atomic mass of the element.

OR

Mass of 6.022×10^23 atoms of an element is define as gram atomic mass or GAM.

OR

Mass of one mole atoms of an element is called GAM.

For example for Sodium atom:

Atomic mass of Sodium (Na) atom = mass of one 'Sodium"atom = 23 amu

GAM= mass of 6.022×10^23 "Na" atoms 

23 amu × 6.022×10^23 "Na" atoms 

GAM= 23  × 6.022×10^23 × 1.67 ×10 ^-24 gram

GAM = 23 amu

Note : (6.022×10^23 × 1.67 ×10 ^-24  =1)

Average atomic mass of magnesium is 24.31 amu. This magnesium is composed of 79 mole % of 24 Mg and remaining 21 mole % of 25Mg and 26 Mg. Calculate mole % of 26Mg.

If an element Z exist in two isotopic form Z^50 and Z^52. The average atomic mass of Z is 51.7. Calculate the abundance of each isotopic forms