Er doped GaN (Er:GaN) is a very promising gain medium for realizing high energy lasers (HELs) operating in the relatively eyesafe 1.5 μm spectral region due to its high thermal conductivity, low thermal expansion coefficient, low temperature coefficient of the refractive index, and high atmospheric transmittance. We report the results of optical absorption and resonantly excited photoluminescence emission spectroscopy studies performed on Er:GaN freestanding quasi-bulk crystals grown by hydride vapor phase epitaxy. Fine features resulting from the transitions between Stark sublevels in the 4I13/2 first excited state and 4I15/2 ground state manifolds enabled the construction of energy level diagrams pertaining to the excitation and emission mechanisms of Er:GaN eyesafe HELs. Our results suggest that the most appropriate pump lines in Er:GaN are 1514 nm and 1539 nm, whereas the lasing emission lines are most likely to occur at 1569 nm and 1581 nm, conforming to the requirements of an extremely small quantum defect lasing system. In contrast to the more established HEL gain medium of Er:YAG, the well-known absorption (or pump) line near 1470 nm is absent in Er:GaN. Er:GaN HELs are expected to outperform those based on Er:YAG in terms of average power, power density, and beam quality.