Theoretical calculations based on the density functional theory, using the PBE functional with the D3 dispersion correction under periodic boundary conditions, have been employed aiming to investigate the properties of α-, β-, and γ-glycine. Structural parameters have been predicted with a maximum error of 1.42% for lattice parameters and 2.53% for the unit-cell volume, for the α phase. Band structure calculations suggest the band gap values of 4.80, 5.01, and 5.23 eV for the α, β, and γ phases, respectively. Quasi-harmonic calculations have been performed and the Gibbs free energy function has been calculated in a wide range of temperature and pressures, suggesting the stability ordering γ > α > β, at room temperature, and the γ to α-glycine phase transition temperature of 442.55 K, at 1 bar, in agreement with the experimental findings. Moreover, a deviation from the experimental value of only 0.44 J mol–1 K–1 is observed for the predicted S(α→γ) at 298.15 K. Finally, calculated sublimation enthalpies of 140.58, 138.09, and 141.70 kJ mol–1 (α, β, and γ-glycine, respectively), at 298.15 K and 1 bar, have also shown good agreement with the experimental values.