1. A system comprising: at least one computing device configured to tune a set of gas turbines (GTs) by performing actions including: commanding each GT in the set of GTs to a base load level, based upon a measured ambient condition for each GT; commanding each GT in the set of GTs to adjust a respective power output to match a nominal power output value, and subsequently measuring an actual exhaust energy value for each GT; and adjusting an operating condition of each GT in the set of GTs based upon a difference between the respective measured actual exhaust energy value and a nominal exhaust energy value at the ambient condition.