New York State (NYS) faces significant challenges in meeting the Climate Act’s bold goals of 70% renewable energy generation by 2030 and total decarbonization of the electric grid by 2040. Extensive simulations are required to assess the impact of numerous inverter-based resources (IBRs) deployed to the large-scale NYS power grid, aiming to evaluate their dynamic behavior and mitigate any negative interactions with their control schemes. However, the modeling efforts required are huge and the computational burden of large-scale simulations is extensive, and often limited by the capabilities of domain- specific tools. This work addresses these limitations by developing a Functional Mockup Unit (FMU) of Grid- Forming (GFM) Inverters for IBR control and integrating them with an electromechanical phasor-domain power system solver. The proposed FMU facilitates the simulation and parametric studies needed to analyze large- scale IBR usage with significantly improved manual modeling and computational efforts. The paper details the process of developing and FMU model for GFM IBRs, including all relevant control loops implemented in the Modelica language and FMU integrated in OPAL-RT’s ePHASORSIM software. Our FMU models are used to successfully deploy and study the impacts of up to 6, 200+ MVA from IBRs on the 5000-bus NYS transmission system.