Modeling adaptive optics (AO) systems is crucial to understanding their performance and a key aid in their design. The Giant Magellan Telescope (GMT) is planning three AO modes at first light: natural guide star AO, ground-layer AO and laser tomography AO. This paper describes how a modified version of YAO, an open-source general-purpose AO simulation tool written in Yorick, is used to simulate the GMT AO modes. The simulation tool was used to determine the piston segment error for the GMT. In addition, we present a comparison of different turbulence simulation approaches.