Gadolinium (Gd, Z = 64) belongs to the lanthanoid series. Its nearest noble gas is Xenon (Xe, Z = 54). Out of the remaining 10 electrons, two fill the $6s$ orbital ($6s^2$). The expected distribution for the rest of the electrons would be $4f^8$. However, an exactly half-filled $f$-subshell ($f^7$) confers extra stability to the atom. Therefore, one electron shifts to the $5d$ orbital, giving the actual ground state electronic configuration as $[\text{Xe}]4f^7 5d^1 6s^2$.