The theoretical performance of gas turbines operating with pressure-gain combustion such as detonation and constant-volume combustion were evaluated by the cycle analysis. To clarify the unsteadiness of state quantities in the pressure-gain combustors, the time and spatial variations of state quantities were numerically computed for the case of pulse detonation combustor. It was found that the ideal pressure-gain combustion cycle had significantly higher thermal efficiency and specific work than that in the constant-pressure combustion cycle. However, when the adiabatic efficiencies of compressor and turbine and the limitation of turbine inlet temperature were introduced to the analysis the thermal efficiency of pressure-gain combustion cycle was identical to or lower than that of constant-pressure combustion cycle. Especially, the limitation of turbine inlet temperature largely degraded the thermal efficiency of pressure-gain combustion cycle. The numerical results showed the quite large variation of gas velocity and mass flow rate as well as pressure and temperature for the pulse detonation, which should be well considered when coupling the combustor with turbine and compressor. It is also found that although pulse detonation produced high peak values of pressure and temperature however their cycle-averaged values were substantially lower than the peak values.