On the most basic sense,what makes GNS3 special is that the virtual devices are capable of running real IOS Images during simulation. This allows GNS3 users to simulate real-world scenarios with precise accuracy with less room for unexpected events compared to less powerful network simulators *cough*Packet Tracer*cough. It seems finding GNS3 IOS Images is hard! There's a reason for that! This is quite a delicate topic.