Effects of the polarization field on the Er3+ intra-4f shell transitions in GaN have been investigated via comparison of photoluminescence emission spectroscopy studies conducted on Er:GaN and Er:YAG. The dominant optical transitions were compared and analyzed. It was observed that the dominant optical transitions in Er:GaN/Er:YAG are between the Stark levels of the same/different irreducible representations. The unique selection rules in Er:GaN are a consequence of the presence of a net local polarization field acting on Er due to the wurtzite crystal structure of GaN. The results provide useful insights into understanding of the dominant optical transitions and the most probable emission lines to be utilized to achieve lasing in Er:GaN.