G A M A is a modeling and simulation development environment for building spatially explicit agent-based simulations. It can be freely downloaded or built from source, and comes pre-loaded with several models, tutorials and a complete on-line documentation. G A M A allows to instantiate agents from any kind of dataset, including GIS data, and execute large-scale simulations (up to millions of agents); couple discrete or continuous topological layers, multiple levels of agency and multiple paradigms (mathematical equations, control architectures, finite state machines); define rich experiments on models and explore their parameters space for calibration and validation; design rich user interfaces that support deep inspections on agents, user-controlled actions and panels, multi-layer 2D/3D displays and multiple agent aspects.