A method for operating a land- or marine-based multi-spool gas turbine system for generating electrical power to supply a load, the system comprising at least two spools, each one of the at least two spools comprising a shaft (11A, 11B, 11C) and a turbine (T1, T2, T3) mounted on the shaft, the turbine arranged to receive gas at an elevated temperature to be expanded in order to produce mechanical power, one of the turbines is being arranged to have the highest pressure of the turbines (T1, T2, T3), i.e. a highest pressure turbine; at least two compressors (C1, C2, C3), each of the at least two spools comprising one of the at least two compressors (C1, C2, C3), the at least two compressors (C1, C2, C3) arranged to receive gas and to compress the received gas; a first heat generating equipment operable to generate heat and to transfer heat into compressed gas so that the compressed gas turns into gas at an elevated temperature that is expanded in the highest pressure turbine to produce mechanical power to drive the compressor and the generator mounted on the same shaft as said turbine; the at least two spools being in fluid communication with each other, wherein each one of downstream turbines (T2, T3) is being arranged to receive gas from a turbine arranged to operate at higher pressure than said turbine receiving the gas; at least two generators (G1, G2, G3), each of which being mechanically coupled to one of the shafts (11A, 11B, 11C) to be rotatably driven thereby, each one of the shafts (11A, 11B, 11C) thus having one mechanically coupled generator, the at least two generators (G1, G2, G3) being operable to generate electrical current to supply the load, and the at least two generators (G1, G2, G3) being independently controllable of each other; and a control equipment (CTRL) arranged to control the operation of the gas turbine system; characterized in that, the method comprises controlling rotational speeds of the at least two generators (G1, G2, G3) independently of each other so as to directly control rotational speeds of the shafts (11A, 11B, 11C) of the spools; and regulating the amount of heat generated in the first heat generating equipment in order to produce the desired output power while maintaining the inlet temperature of the highest pressure turbine substantially at the predefined maximum allowable value in a first control mode; and controlling the rotational speeds of the at least two generators (G1, G2, G3) at predefined substantially constant values and regulating the amount of heat generated in the first heat generating equipment in a second control mode, wherein the method further comprises utilizing the first or the second control mode based on an output power of the system. controlling rotational speeds of the at least two generators (G1, G2, G3) independently of each other so as to directly control rotational speeds of the shafts (11A, 11B, 11C) of the spools; and regulating the amount of heat generated in the first heat generating equipment in order to produce the desired output power while maintaining the inlet temperature of the highest pressure turbine substantially at the predefined maximum allowable value in a first control mode; and controlling the rotational speeds of the at least two generators (G1, G2, G3) at predefined substantially constant values and regulating the amount of heat generated in the first heat generating equipment in a second control mode, wherein the method further comprises utilizing the first or the second control mode based on an output power of the system.