The important information or constant, in this case, to know is the Avogadro's constant. The constant is equal to 6.02214086 × 10^23 atoms/mol. Multiply this constant with the number of moles a gram of gold would contain.
= 1gram/(196.96g/mole)*6.022× 10^23 atoms/mol = 3.057*10^21 atoms.