A gas turbine engine system (10, 100, 150) comprising: a gas turbine engine (12, 112) comprising: a compressor section (18, 118) comprising: a low pressure compressor (20, 120) configured to compress an inlet air flow (Fi) to produce a first compressed air flow; a high pressure compressor (22, 122) fluidly coupled to the low pressure compressor (20, 120) and configured to compress the first compressed air flow to produce a second compressed air flow; a combustor (24, 124) fluidly connected to an outlet of the high pressure compressor (22, 122) and configured to combust a mixture of the second compressed air flow and a gaseous fuel at a first pressure to produce a combustion gas flow; a turbine fluidly coupled to the combustor (24, 124) 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 configured to transfer thermal energy from the first compressed air flow and the exhaust gas flow (F E) to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure; and a fuel turbine system comprising: a fuel turbine fluidly coupled to the heat exchange system and the combustor (24, 124), 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 fluidly coupled to the heat exchange system and configured to be driven by the fuel turbine; characterised in that: the fuel turbine is a multi-stage turbine; and the fuel turbine includes an interstage discharge outlet configured to deliver gaseous fuel at the first pressure to the combustor, wherein the first pressure is greater than a pressure of the second compressed airflow. a gas turbine engine (12, 112) comprising: a compressor section (18, 118) comprising: a low pressure compressor (20, 120) configured to compress an inlet air flow (Fi) to produce a first compressed air flow; a high pressure compressor (22, 122) fluidly coupled to the low pressure compressor (20, 120) and configured to compress the first compressed air flow to produce a second compressed air flow; a combustor (24, 124) fluidly connected to an outlet of the high pressure compressor (22, 122) and configured to combust a mixture of the second compressed air flow and a gaseous fuel at a first pressure to produce a combustion gas flow; a turbine fluidly coupled to the combustor (24, 124) 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 configured to transfer thermal energy from the first compressed air flow and the exhaust gas flow (F E) to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure; and a compressor section (18, 118) comprising: a low pressure compressor (20, 120) configured to compress an inlet air flow (Fi) to produce a first compressed air flow; a high pressure compressor (22, 122) fluidly coupled to the low pressure compressor (20, 120) and configured to compress the first compressed air flow to produce a second compressed air flow; a low pressure compressor (20, 120) configured to compress an inlet air flow (Fi) to produce a first compressed air flow; a high pressure compressor (22, 122) fluidly coupled to the low pressure compressor (20, 120) and configured to compress the first compressed air flow to produce a second compressed air flow; a combustor (24, 124) fluidly connected to an outlet of the high pressure compressor (22, 122) and configured to combust a mixture of the second compressed air flow and a gaseous fuel at a first pressure to produce a combustion gas flow; a turbine fluidly coupled to the combustor (24, 124) 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 configured to transfer thermal energy from the first compressed air flow and the exhaust gas flow (F E) to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure; and a fuel turbine system comprising: a fuel turbine fluidly coupled to the heat exchange system and the combustor (24, 124), 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 fluidly coupled to the heat exchange system and configured to be driven by the fuel turbine; a fuel turbine fluidly coupled to the heat exchange system and the combustor (24, 124), 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 fluidly coupled to the heat exchange system and configured to be driven by the fuel turbine; characterised in that: the fuel turbine is a multi-stage turbine; and the fuel turbine includes an interstage discharge outlet configured to deliver gaseous fuel at the first pressure to the combustor, wherein the first pressure is greater than a pressure of the second compressed airflow. the fuel turbine is a multi-stage turbine; and the fuel turbine includes an interstage discharge outlet configured to deliver gaseous fuel at the first pressure to the combustor, wherein the first pressure is greater than a pressure of the second compressed airflow.