Gadolinium has a low value of third ionisation enthalpy because of:

The electronic configuration of Gadolinium ($\text{Gd}$) is $[\text{Xe}]4f^{7}5d^{1}6s^2$.Removing the first two electrons from $6s$ and the third from $5d$ leaves a stable, exactly half-filled $4f^7$ orbital. The stability associated with the half-filled configuration is due to high exchange enthalpy. Since the resulting state is highly stable, the energy required to reach it (the 3rd ionisation enthalpy) is lower than expected.