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)