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 [Xe]4f76s2. 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  [Xe] 4f75d16s2.