GaiA is a simulation tool for autonomous driving and ADAS development validation. It can restore complex roads by integrating the road network database, and can reproduce the realistic driving environment through the use of the environment building model library. GaiA provides rich C++ and matlab interfaces, which can be applied to various driving vehicles and systems under test. GaiA can generate a large number of traffic participants, and manually and automatically set up its traffic behavior planning, and even change the aggressiveness of driving behavior. GaiA also provides high-fidelity environmental perception sensors, including millimeter-wave radar, lidar, cameras, and more.