System engineering at GMTO is using a comprehensive integrated model that integrates seamlessly, in a unified framework, finite element, optics, and control models. A computational fluid dynamics (CFD) model of the observatory is also used to estimate dome seeing, wind jitter, structural thermal deformations, and observatorywide design optimization. The GMT integrated modeling group realizes various studies for different subsystems of the project that provides the basis for the subsystem level design trades. It also assists system engineering by performing top-down and bottom-up requirements verification, error budget derivation, and operational strategies optimization. Integrated modeling will also support system engineering during the assembly, integration, verification, and commissioning phase of the project. For example, system engineering relies on the integrated model to estimate the key performance parameters (KPP) of the project. The KPP are performance metrics that will be used to validate the completion of the observatory and to confirm its readiness with respect to the start of science observation. In the paper, we give a system-level overview of the integrated model, including a description of each sub-model and of the framework that binds them together. The paper also describes how system engineering is using the integrated model for the derivation of the error budgets and of the top-down requirements flowing down from the science requirements to the lower level of subsystem engineering requirements; and how as the design of the subsystems progress, integrated modeling is then used to validate, bottom-up, the same requirements from subsystem engineering requirements back up to the science requirements with respect to the observatory performance metrics.