Gadolinium has a low value of third ionisation enthalpy because of

Exchange energy is the energy released when two or more electrons with the same spin exchange their positions in the degenerate orbitals of a subshell. On losing third ${\mathrm{e}}^{-}$, Gadolinium achieves half filled $4\mathrm{f}$-orbital configuration, which has higher exchange enthalpy.

$\mathrm{Gd}=\left[\mathrm{Xe}\right] 4{\mathrm{f}}^{7}5{\mathrm{d}}^{1}6{\mathrm{s}}^2$.