Extended x-ray absorption fine structure and photoluminescence studies were performed on epitaxial GaN films implanted with 1·1016 cm-2 Er ions at 80 and 160 keV and, for a part of the samples, co-implanted with oxygen ions at 23 keV, followed by an anneal for 60 min at 900°C. It was shown for the samples both with, as well as without, oxygen co-implantation that Er is incorporated in a six-fold coordination with respect to oxygen, as in the cubic bixbyite structure Er2O3. The oxygen contamination of the non-oxygen-implanted samples is assumed to be due to nitrogen-vacancy-assisted oxygen diffusion from the sapphire substrate during annealing. The Stark level splitting of the 4I15/2 ground state of Er3+ observed in the 1.54 µm photoluminescence at low temperature in both types of samples is consistent with the low symmetry of the Er sites expected in cubic bixbyite Er2O3.