That looks good, and yes you should use same INCAR.  FYI, you only need gamma kpoints for H2 calculations. Also, it's recommended to decrease the width of smearing (sigma) for gas phase. I typically use 0.01 for gas phase.

Thanks for your kind suggestion. can you please tel me POSCAR to be used that is should I use H2 in a box of 7x7x7Angstrom cube?

You can likely find these information in other publications. I would recommend looking up papers in your field and see what they did. I think people use 20x21x22 ang cube as standard.

@Geun Ho Gu, Thanks for your kind reply I would like to ask the following

I have the relaxation energy of MgH2 bulk containing 6 atoms (2 Mg , 4 Hydrogen). The energy is -17.632 ev and for Mg Bulk (Having 6 Mg atoms) is -6.025 ev. The H2 molecule in 14x14x14 box have relaxation energy of -6.795 ev. Now as mentioned above the reaction is MgH2 = Mg+ H2. 

For enthalpy I have applied the formula

Enthalpy of desorption of MgH2 =E(MgH2)-E(Mg)-E(H2).

but my answer is not correct . The enthalpy as reported in other papers should be -74.60 Kj/mol.

I think I have to normalize my Enthalpy formula wrt no of atoms. so what should be the correct formula in terms of number of atom in bulk taken in to account?

Thanks

I don't research materials, so I don't know the definition of enthalpy of desorption in the context of materials science. There likely be a defined equation for this. But from what I can see, it seems that you did not match the stoichiometry. i.e. it should be H_des(MgH2) = 1/2*E(Mg2H4) - 1/6*E(Mg6) - E(H2) = -1.02 eV = 98.42 kJ/mol. It's closer to -74.60 kJ/mol. ~25 kJ/mol difference could be caused by many different factors such as your input settings (functional, k-points, etc.)

It also doesn't logically make sense why this energy would be called enthalpy of desorption. Vibrations(or phonon) are unaccounted and the equation is defined in the direction of hydrogen adsorption. I would recommend looking up papers for the definition first, and then, compare computation details.