The crystallized amino acid γ-glycine is a large band gap insulator that shows promise in the fields of photonics and non-linear optics. To better understand its physical properties, the effect of pressure on the structural, electronic and optical properties of γ-glycine were investigated through a first principles calculation approach based on density functional theory. A band gap of 5.026 eV was found and was shown to decrease with an increase of pressure, due to the widening of the conduction and valence bands. The densities of states verify the observations made in the band structure and exhibit that the conduction bands are dominated by the O 2p and C 2p orbitals. The absorption spectra for γ-glycine was calculated using many-body Green's functions GW and reveals a slight blueshift in the absorption spectra. The information presented in this paper might be useful in better understanding the structural and optical properties of γ-glycine for future application.