The structure of gadolinium oxyfluoride nanoparticles was revised. Extensive studies including X-ray diffraction and Rietveld refinement as well as Fourier transform infrared spectroscopy and Raman spectroscopy confirmed the monoclinic P12/c1 crystal structure of Gd4O3F6. Morphological analysis using transmission electron microscopy showed the nanocrystallinity of the materials prepared via the sol-gel Pechini's method. The luminescent properties of the prepared materials with different concentrations of Eu(3+) ions were characterized by emission spectroscopy. The phosphors obtained were investigated in the vacuum ultraviolet range using synchrotron radiation. The Judd-Ofelt parameters (Ω2, Ω4) and emission efficiencies η were calculated and are discussed in detail.