•A gas turbine combined cycle with gasified biomass external firing is analyzed.•Thermodynamic analysis considers first and second law analyses.•Thermal efficiency peaks at an optimum cycle pressure ratio (about 9).•Three sets of operating parameters are considered in a detailed case study.•One of three cases is more efficient from first or second law viewpoints. Thermodynamic analyses of an externally fired gas turbine combined cycle integrated with a biomass gasification plant, using energy and exergy approaches, are reported for a proposed configuration. Paper is taken to be the fuel and the thermodynamic performance and sizing of the plant is examined for various values of cycle pressure ratio (7–12), gas turbine inlet temperature (1200–1400K) and heat exchanger cold-end temperature difference (245–275K). Depending on the gas turbine inlet temperature and heat exchanger cold-end temperature difference, the system overall energy efficiency is observed to attain a maximum at a particular pressure ratio. For a given pressure ratio, the energy efficiency increases with gas turbine inlet temperature and decreases with heat exchanger cold-end temperature difference. An increase in pressure ratio results in a decrease of air flow rate and an increase of steam flow rates. These flow rates are attributable to the size of combined cycle plant. Raising the gas turbine inlet temperature reduces the air flow rate. The performance of a 1MW plant is investigated with various operating parameters to obtain component ratings and biomass feed rates. Exergy efficiencies of cycle components are examined along with the major thermodynamic irreversibilities.