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

Due to more stable exactly half-filled $4{\mathrm{f}}^7$ shell, electron filling occur in a little higher energy $5\mathrm{d}-$orbital

So configuration of ${\mathrm{Gd}}^{2+}=\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}5{\mathrm{d}}^1$