Electronic configuration of ${}_{64}$Gd: [Xe] 6s${}^2$ 4f${}^7$ 5d${}^1$.

Removing two electrons gives Gd${}^{2+}$: [Xe] 4f${}^7$ 5d${}^1$. The third ionisation removes the 5d${}^1$ electron, leaving Gd${}^{3+}$: [Xe] 4f${}^7$, a half-filled 4f subshell.

A half-filled 4f subshell is exceptionally stable because of high exchange energy. This extra stability of the Gd${}^{3+}$ product makes the third ionisation enthalpy unusually low.