Abstract: A gas turbine for a gas and steam turbine plant and method for operating a gas turbine is provided. The gas turbine includes a cooling system for cooling inlet air taken from compressor extractions or compressor discharge and introduced to the turbine(s) and/or combustor(s) of the gas turbine which is adapted for flexible operation in a combined cycle application as well as a simple cycle application. The cooling system includes at least a water/steam cooled cooling air cooler for use in a normal combined cycle application. The gas turbine is provided with at least one second cooling air system arranged in a bypass which is operable instead of or parallel to the water/steam cooled cooling air cooler for use in particular in a simple cycle application operation of the gas turbine.