Vacancy-type defects in Mg-implanted GaN were probed using monoenergetic positron beams. Mg+ ions were implanted to provide box profiles with Mg concentrations [Mg] of 1017-1019 cm-3. For as-implanted samples, the major defect species was determined to be Ga-vacancy (VGa) related defects such as divacancy (VGaVN) and/or their complexes with impurities. For Mg-implanted samples, an agglomeration of vacancies started at 800-1000°C annealing, leading to the formation of vacancy clusters such as (VGaVN)3. For the sample with [Mg]=1019 cm-3, the trapping rate of positrons to the vacancies decreased with increasing annealing temperature (≥1100°C), which was attributed to the change in the charge state of vacancy-type defects from neutral to positive (or negative to neutral) due to the activation of Mg. For Mg- and H-implanted samples, the hydrogenation of vacancy-type defects started after 800°C annealing. Comparing with the annealing behavior of defects for the samples without H-implantation, the clustering of vacancy-type defects was suppressed, which can be attributed to the interaction between Mg, H, and vacancies.