For gas turbines (GTs) with free power turbines (FPTs), the capacity or flow parameter matching is of prime importance. Accurately matched capacity enables the GT to run at its optimum condition. This ensures maximum component efficiencies and optimum shaft speeds within mechanical limits. This paper presents the challenges, uncertainties, and opportunities associated with an accurate matching of a generic two-shaft aeroderivative high pressure (HP)-low pressure (LP) gas generator with the FPT. Additionally, generic performance trends, uncertainty quantification, and results from the verification program are also discussed. These results are necessary to ensure that the final FPT capacity is within the allowable range, and hence, the product meets the performance guarantees. The sensitivity of FPT capacity to various design variables such as the vane throat area, vane trailing edge size, and manufacturing tolerance is presented. In addition, issues that may arise due to not meeting the target capacity are also discussed. To conclude, in addition to design, analysis, and statistical studies, a system-of-systems approach is mandatory to meet the allowed variation in the FPT capacity and hence the desired GT performance.