What is the difference between 1g atom of nitrogen and 1 g of nitrogen

"Gram of atoms" is in the normal English sense. If you say 10 grams OF atoms, it can be 10 grams of any atoms. Could be 10 grams of Hydrogen atom or Oxygen atoms or Na atoms. And the number of atoms in each of this samples will be different.
When u say "gram atoms" it is a technical term. It means a particular number of atoms. This number is called Avogadro Number 6.022 X 10^23. If u take this many number of Oxygen atoms, it will weigh 16 grams. If u take this many number of Nitrogen atoms, it will weigh 14 gram. An amount in grams which is numerically equal to the mass number of the element.