Gadolinium (Gd) has an atomic number of $64$. The nearest noble gas is Xenon ($\text{Xe}$, $Z = 54$). The remaining $10$ electrons are to be filled in the $4f$, $5d$, and $6s$ orbitals. The general electronic configuration of lanthanoids is $[\text{Xe}] 4f^{1-14} 5d^{0-1} 6s^2$. To acquire the extra stability associated with a half-filled $f$-subshell ($4f^7$), one electron enters the $5d$ orbital instead of pairing up in the $4f$ orbital. Therefore, the ground state electronic configuration of Gadolinium is $[\text{Xe}] 4f^7 5d^1 6s^2$.