In this article, the optimal gain value of classical controller gain values is obtained by using different performance indices in Automatic Generation Control. These are interconnected by the three areas of a hydro-thermal power system. The thermal and hydro areas are incorporated with a reheat turbine and a mechanical governor, respectively. The current article was created to select a suitable technique for the tuning of controller gain, when 1% of a step load is given to the thermal area (Area 1). The performance of several controllers, such as Integral (I), Proportional-Integral (PI), and Integral-Derivative (ID) were evaluated and were compared to the cases of with and without Generation Rate Constraint (GRC) non-linearity. The classical controller gain value optimization was performed using the Integral Time Square Error (ITSE), Integral Square Error (ISE) and Integral Time Absolute Error (ITAE) performance indices. The minimum optimal value of controller gain normally offers better dynamic response. The obtained results established that the optimization based on the ITAE-PI controller always guarantees superior dynamic performances compared to other indices and controllers.