In this paper, a performance optimisation based on the exergetic/ ecological coefficient of performance (ECOP) criterion has been performed for an irreversible simple, regenerative and cogeneration gas turbine systems by using exergy analysis method. The ECOP objective function defined as the ratio of exergy output to the loss rate of availability (or total exergy destruction). The obtained results were matched against those obtained using the power output and exergy efficiency. The design parameters under the optimal conditions have been derived numerically and their effects on the engine's performance have been discussed. The effect of the design parameters such as pressure ratio of compressor, cycle temperature ratio, isentropic efficiencies of turbine and compressor, effectiveness of the regenerator and pinch point temperature difference have also been investigated. The results obtained from this paper will provide guidance for the design of simple, regenerative and cogeneration gas turbine systems and can be used for selection of optimal design parameters.