Photomodulated reflection spectroscopy in combination with the diamond cell technique has been used to study the pressure dependence of the E0 and E1 gaps in GaAs. In addition, the pressure-volume (PV) relation up to 170 kbar is determined by high pressure x-ray diffraction. The dependence of the optical transition energies on volume change (linear and quadratic coefficients) as well as the experimental PV relation are compared to results of selfconsistent relativistic LMTO-ASA band structure calculations