What is meant by atomic mass, gram atomic mass of an element? Why is the mass have different expressions i.e., 'u' and 'g'?

The atoms are very tiny and their individual mass cannot be calculated as it is negligible. Hence the mass of atoms is expressed in units with respect to a fixed standard. Initially hydrogen atom with mass 1 was taken as standard unit by Dalton. Later, it was replaced by oxygen atom (0 = 16). But due to the isotopes the masses were found in fractions instead of whole number. Hence, carbon (C = 12) isotope was taken as standard unit and was universally accepted.

The atomic mass unit is equal to one twelfth (1/12) the mass of an atom of carbon-12, its unit is u.

Gram atomic mass: When the atomic mass of an element is expressed in grams, it is called the gram atomic mass of the element.

The mass of atoms, molecules is expressed in 'u' and the mass of moles i.e., molar mass is expressed in g.