Currently, the usefulness of gas turbines in the power generation business cannot be undermined. But to get the most value from their use, they should be operated in a way to maximise power output, minimise fuel consumption with increased thermal efficiency and minimal negative impact on the environment. This research aims at improving the power output of GT 20, which will also help to improve its performance; and will be significant at providing more power to Nigeria’s national grid from the unit for increased production activities. Experimental plants GT 20 and GGTPP were compared with a model plant used to simulate their behaviour. The results of the study show that GT 20 maximum power output from its operation was 92MW with efficiency of 0.34 at a pressure ratio of 10.27. Comparison made between GT 20 and the two other power plants – GGTPP and the model indicate that the low performance of GT 20 was due to unfavourable operating conditions of the unit, but the unit is a much older plant. A mix of optimisation variables of mass flow, compressor inlet temperature, turbine inlet temperature, and pressure ratio were applied to the model and used to optimise GT 20 and its power output. The plant has been optimised at a power output of 100.07 MW and an efficiency of 0.35, with improvements in its performance measures by 5%, 2.94%, and 8.77% in specific fuel consumption, overall efficiency and power output respectively.