Erbium doped GaN (Er:GaN) is a promising candidate as a new gain medium for high energy lasers. The excitation and emission mechanisms as well as the transition cross sections of the pump and laser wavelength are of paramount importance for understanding the performance of lasers and amplifiers made of Er:GaN materials. We report here the results of direct measurements of resonantly excited photoluminescence emission, photoluminescence excitation, and optical absorption spectroscopy in the 1.5 μm "retina-safe" spectral region performed on freestanding Er:GaN bulk crystals synthesized by hydride vapor phase epitaxy. The results established that 1514 nm and 1538 nm are the most appropriate resonant pump wavelengths for achieving gain and lasing, which differs from Er in YAG and glass hosts. The absorption coefficients (α) and absorption cross-sections (σexc) of Er in GaN in the 1.5 μm window have been directly measured, providing σexc = 1.4 × 10-20 cm2 and 2.7 × 10-20 cm2 for 1514 nm and 1538 nm pump wavelengths, respectively. These values are considerably higher than those of Er ions in glass and YAG hosts.