Large-sized, high optical quality Gd2O3 single crystals were grown from a solution at T = 1145 °C using Czochralski method. XRD analysis showed that the crystal structure is characterized by a single cubic (bixbyite) phase (c-phase). The absorption spectra and luminescence properties of Gd2O3 doped with Eu3+ and Tb3+ impurity ions were studied. The absorption spectrum of Gd2O3:Eu in the short wavelength region is mainly due to charge transfer transitions from O2− to Eu3+ (240–290 nm), which overlap with the Urbach tail of the host-absorption. Additionally, electronic transitions in Eu3+ (320–540 nm) and Gd3+ (245–315 nm) lanthanide ions contribute to the absorption spectrum. Intraconfigurational radiative f – f transitions in impurity ions are clearly observed upon both UV- and X-ray excitations. However, 6PJ → 8S7/2 radiative transitions in Gd3+ ions are completely unobservable. Based on the obtained spectroscopic data, the energy positions of the ground state of all di- and trivalent lanthanide ions, as well as the energies of intraconfigurational f – f and interconfigurational f – d electronic transitions in Gd2O3 crystals with a cubic structure, are calculated. The proposed electronic energy diagram is used to discuss efficient energy transfer between lanthanide ions.