Anarchy is an engine used for GA1, GA2, GA3 and GADO.
The Anarchy 1 engine was written for Global Anarchy 1. It used a modified version of the SE1 game engine, and a 3d doom engine written by orange451 in 2010.
The Anarchy 2 engine was written for Global Anarchy 2. It used a modified version of the Anarchy 1 engine, but also used p3dc, a 3d polygonal collision detection dll, which allowed for more complex terrain with less processing power.
The Anarchy 3 engine was written for Global Anarchy 3. GADO uses the Anarchy 3 engine as a base for it's game, but it also add's some optimised features from the GA3 game itself.