A #"gram atom"# is a bit of an old-fashioned term, that refers to the mass of #"Avogadro's number"# of #"atoms"#............i.e. a gram atom of carbon is #12*g#, of oxygen is #16*g#, of hydrogen is #1*g#.
And each of these #"gram atoms"# specifies precisely #"Avogadro's Number"# of atoms, i.e. #6.022xx10^23*mol^-1#.