In the gaseous state, the atoms are widely separated therefore, interatomic forces are minimum. It is because of this reason the term ‘isolated gaseous atom’ has been included in definition of ionisation enthalpy and electron gain enthalpy.
The term ‘ground state’ here means that the atom must be present in the most stable state, i.e., the ground state. Therefore, for comparison purposes, the ionisation enthalpies and electron gain enthalpies of gaseous atoms must be determined in their respective ground states.