Positron annihilation is a non-destructive technique for investigating vacancy-type defects in condensed matter. When a positron is implanted into a sample, it annihilates with an electron and emits two 511-keV γ quanta. From measurements of Doppler broadening spectra of the annihilation radiation and positron lifetimes, one can detect point defects such as monovacancies, vacancy clusters, and vacancy-impurity complexes. The regions sampled can range from the surface to a depth on the order of microns. In the present study, we have used the positron annihilation technique to study relationship between the impurity doping and vacancies in GaN. Defects in ion-implanted GaN and their annealing properties were studied. The defects introduced by the implantation were identified as divacancies, and the defect reaction during isochronal annealing were found to depend on ion spices. A relationship between intra-4f transitions of Er and vacancies was studied. A correlation between the defect concentration and the PL intensity was observed. We will demonstrate that the positron annihilation technique is sensitive to vacancy-type defects in GaN, and it can contribute to the development of optical and electronic devices based such materials.