⁕ Gram Atomic Mass (g atom)

↪ Atomic mass is simply a number, which does not have any unit of mass.
↪ The atomic mass or simply gram atom is the atomic mass expressed in terms of grams.
↪ Therefore, 1 gram atom of hydrogen weighs 1 gram.
Similarly 1 gram of Oxygen, Sulphur and Sodium weigh 16g, 32g and 23g respectively.

Number of gram atom of element = 

Further, gram atom is denoted as mole of atoms.

⁕ Gram Molecular Mass (g mole)

↪ The molecular mass is a relative term and it is a unit less number.
e.g. molecular mass of H₂SO₄ = 98.
↪ It means, the weight of 1 molecules of H₂SO₄ is 98 times heavier than the mass of 1/12th of an atom of C-12 isotope (i.e. 1 amu).
↪ When the molecular mass of substance is expressed in gram, it is called one gram molecular mass or 1 g mole.

For example:
Molecular mass of H₂ = 2 amu and 2 g of H₂ = 1 g mole of H₂
Molecular mass of CO₂ = 44 amu and 44 g of CO₂ = 1 g mole of CO₂

Mathematically, we have
No. of mole = 

Further, g mole is denoted as mole of molecules.