Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
RunGEM is a freeware component of the commercial modelling package, Gempack, written at Monash University.
It is mainly used for solving (ie, simulating with) computable general equilibrium (CGE) models. It does not allow a user to change a model's specification or to create a new model.