A gas turbine engine system (10, 50) comprising: a gas turbine engine (12, 52) comprising: an air inlet configured to receive an inlet air flow (Fi); a compressor configured to compress the inlet air flow (Fi) to produce a compressed air flow; a combustor fluidly coupled to the compressor and configured to combust a mixture of the compressed air flow and a gaseous fuel at a first pressure to produce a combustion gas flow; a turbine fluidly coupled to the combustor and configured to extract energy from expansion of the combustion gas flow to produce an exhaust gas flow (F E); and a heat exchange system (16,36,54,56) configured to transfer thermal energy from an air flow to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure, wherein the air flow is the inlet air flow (F I) or the exhaust gas flow (F E); and a turbo-generator comprising: a fuel turbine fluidly coupled to the heat exchange system and the combustor, wherein the fuel turbine is configured to extract energy from expansion of the gaseous fuel at the second pressure to produce the gaseous fuel at the first pressure; and a fuel pump configured to be driven by the fuel turbine, wherein the fuel pump is fluidly coupled to the heat exchange system; characterised in that: the turbo-generator further comprises a motor/generator comprising a cooling jacket, wherein the motor/generator is configured to be driven by the fuel turbine, the cooling jacket is fluidly coupled to the fuel pump, and the cooling jacket is fluidly coupled between a fuel tank and the fuel pump. a gas turbine engine (12, 52) comprising: an air inlet configured to receive an inlet air flow (Fi); a compressor configured to compress the inlet air flow (Fi) to produce a compressed air flow; a combustor fluidly coupled to the compressor and configured to combust a mixture of the compressed air flow and a gaseous fuel at a first pressure to produce a combustion gas flow; a turbine fluidly coupled to the combustor and configured to extract energy from expansion of the combustion gas flow to produce an exhaust gas flow (F E); and a heat exchange system (16,36,54,56) configured to transfer thermal energy from an air flow to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure, wherein the air flow is the inlet air flow (F I) or the exhaust gas flow (F E); and an air inlet configured to receive an inlet air flow (Fi); a compressor configured to compress the inlet air flow (Fi) to produce a compressed air flow; a combustor fluidly coupled to the compressor and configured to combust a mixture of the compressed air flow and a gaseous fuel at a first pressure to produce a combustion gas flow; a turbine fluidly coupled to the combustor and configured to extract energy from expansion of the combustion gas flow to produce an exhaust gas flow (F E); and a heat exchange system (16,36,54,56) configured to transfer thermal energy from an air flow to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure, wherein the air flow is the inlet air flow (F I) or the exhaust gas flow (F E); and a turbo-generator comprising: a fuel turbine fluidly coupled to the heat exchange system and the combustor, wherein the fuel turbine is configured to extract energy from expansion of the gaseous fuel at the second pressure to produce the gaseous fuel at the first pressure; and a fuel pump configured to be driven by the fuel turbine, wherein the fuel pump is fluidly coupled to the heat exchange system; a fuel turbine fluidly coupled to the heat exchange system and the combustor, wherein the fuel turbine is configured to extract energy from expansion of the gaseous fuel at the second pressure to produce the gaseous fuel at the first pressure; and a fuel pump configured to be driven by the fuel turbine, wherein the fuel pump is fluidly coupled to the heat exchange system; characterised in that: the turbo-generator further comprises a motor/generator comprising a cooling jacket, wherein the motor/generator is configured to be driven by the fuel turbine, the cooling jacket is fluidly coupled to the fuel pump, and the cooling jacket is fluidly coupled between a fuel tank and the fuel pump.