Magnetic moment of ${\mathrm{Gd}}^{3+}$ion (Z = 64) is

$$\begin{gathered} \mathrm{Gd}\left(64\right)=\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}5{\mathrm{d}}^{1}6{\mathrm{s}}^2 \\ {\mathrm{Gd}}^{3+}=\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}5{\mathrm{d}}^{0}6{\mathrm{s}}^0 \\ \text{ i.e. no. of unpaired electrons }=7 \\ \mathrm{\mu }=\sqrt{\mathrm{n}(\mathrm{n}+2)}=\sqrt{7(7+2)} \\ = \sqrt{63}=7.93\mathrm{BM} \end{gathered}$$