The synthesis of the rhombohedral Gd6UO12-δ is reported via mechanochemical processing of stoichiomet ric Gd2O3/UO2 mixtures and their subsequent annealing. Rietveld refinement of XRD data reveals that the as-prepared material exhibits a remarkable degree of cation antisite disorder and oxygen deficiency. The simulations of intensities of the selected XRD superlattice reflections are performed for limiting states of Gd6UO12-δ with its most extreme degrees of the cation antisite disorder. On the basis of the estimated bond lengths it can be stated that distorted geometry of structural units in the material is a consequence of its relatively large oxygen deficiency.