Thick freestanding iron-doped semi-insulating GaN layers were grown by Hydride Vapor Phase Epitaxy on GaN/sapphire templates. Iron doping was achieved by using Fe-57-enriched Fe2O3 reduced to elemental Fe in-situ avoiding uptake of C from organometallic sources. The morphology and crystalline quality of the films show no evidence of degradation upon iron doping. Mossbauer and spin resonance experiments demonstrate that the Fe-impurity is in the isolated Fe3+ paramagnetic state and no Fe-precipitates are formed at the highest doping levels. Low temperature photoluminescence studies are consistent with full compensation of the shallow pervasive Si and O donors.