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)