The electronic configuration of gadolinium (Atomic number 64) is

The electronic configuration of La (Z = 57) is [Xe]5d¹6s².Therefore, further addition of electrons occurs in the lower energy4f-orbital till it is exactly half-filled at Eu(Z = 63). Thus, the electronic configuration of Eu is $\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}6{\mathrm{s}}^2$. Thereafter addition of next electron does not occur in the more stable exactly half-filled 4f⁷ subshell but occurs in the little higher energy 5d-orbital.
Thus, the electronic configuration of Gd(Z = 64) is $[\mathrm{Xe}] 4{\mathrm{f}}^{7}5{\mathrm{d}}^{1}6{\mathrm{s}}^2.$