Various embodiments include a system having: 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 output to match a nominal mega-watt power output value, and subsequently measuring an actual fuel flow value and an actual emissions value for each GT; adjusting at least one of a fuel flow or a water flow for each GT to an adjusted water/fuel ratio in response to the actual emissions value deviating from an emissions level associated with the base load level, while maintaining the respective adjusted output; and adjusting an operating condition of each GT in the set of GTs based upon a difference between the respective measured actual fuel flow value and a nominal fuel flow value at the ambient condition, while maintaining the adjusted water/fuel ratio.