Question

what is meant by negative value of electron gain enthalpy and positive value of electron gain enthalpy

Answer 1

When an atom gains an electron, energy is either released or absorbed.

If the energy is released, it means that the atom readily accepts electron and thus moves to a more stable state. The release of energy is given a negative sign and hence called as negative electron gain enthalpy.
E.g When oxygen goes from O to O^- it readily accepts electrons due to its electronegative nature and has negative electron gain enthalpy.

On the other hand, when acceptance of electron is difficult and energy is absorbed. it leads to positive electron gain enthalpy.