Gram Molecular Mass:

The molecular mass of a substance (Molecule) express in gram is called gram molecular mass of the substance.

OR

Mass of 6.022×10^23 molecules of a substance is define as gram molecular mass or GMM.

OR

Mass of one mole  of a molecule is called GMM.

For example for O2 molecule:

Molecular mass of O2 molecule = mass of one  OF molecule = 2 × mass of one "O" atom = 2× 16 amu = 32 amu

GMM= mass of 6.022×10^23 "O2" molecules =

32 amu × 6.022×10^23 "O2"  molecules.

GMM= 32  × 6.022×10^23 × 1.67 ×10 ^-24 gram

GMM = 32 amu

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

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.