First-principles density-functional approach was used to obtain the crystal parameters and phonon spectra of gamma-Mg2SiO4 (spinel and ringwoodite) as functions of pressure. The Gruneisen parameters at the Gamma point were obtained and compared with the experimental data. Using quasiharmonic approximation, the total free energy of the crystal was calculated as a function of volume for several temperatures, hence, the bulk modulus and thermal expansion were found as a function of temperature. A good agreement between calculated and experimental values was obtained in a wide temperature range.