Decomposition of gamma-cyclotrimethylene trinitramine (gamma-RDX) under high pressure-high temperature conditions was examined to elucidate the reactive behavior of RDX crystals. Vibrational spectroscopy measurements were obtained for single crystals in a diamond anvil cell (DAC) at pressures from 6 to 12 GPa and temperatures up to 600 K. Global decomposition rates, activation energies, and activation volumes at several pressures and temperatures below the P-T locus for the gamma-RDX decomposition were obtained. Similar to epsilon-RDX, but in contrast to alpha-RDX, we found that pressure decelerates the decomposition of gamma-RDX. The decomposition deceleration with pressure in the gamma-phase can be attributed to pressure-inhibiting bond homolysis step(s). The main decomposition species were identified as N2O, CO2, and H2O, in accord with the species reported for the alpha-phase decomposition at high pressures. This work complements previous studies on RDX at HP-HT conditions and provides comprehensive results on the reactive behavior of gamma-RDX; the gamma-phase plays a key role in RDX decomposition at P-T conditions relevant to shock wave initiation.