The atomic mass of an element expressed in grams is called gram atomic mass of that element.
or
It is also defined as mass of 6.022 x 1023 atoms.
or
It is also defined as the mass of one mole atoms.
For Example for oxygen atom:
Atomic mass of ‘O’ atom = mass of one ‘O’ atom = 16 amu
Gram atomic mass = mass of 6.022 × 1023 ‘O’ atoms
= 16 amu x 6.022 × 1023 = 16 x 1.66 × 10-24 x 6.022 x 1023 = 16 gm/mole
Note:
1.66 x 10-24 x 6.022 x 1023 = 1