Precise performance evaluation at design and off-design operations is needed for a correct management of power plants. This need is particularly strong in gas turbine power plants which can quickly react to load variations and are very sensitive to ambient conditions. The paper aims at presenting a simple tool to determine the values of the thermodynamic quantities in each point of the plant and the overall plant performances of a real gas turbine plant. Starting from experimental data, a zero-dimensional model is developed which properly considers the effect of ambient conditions and water injection for pollutant abatement at different load settings under the action of the control system. An emission model taken from the literature is also included, after tuning on experimental data, to predict carbon monoxide and nitrogen oxide pollution