The Gamma reality is a subset of the Beta reality in which the habitat is treated as a simulator for the purposes of training. This is not a breaking of the Golden Rule.
Use for training
As the mission is extremely complex, members must train by doing, in order to develop farmiliarity with equipment and procedures. As such, training missions are held in the gamma reality.
In the gamma reality, participants will act as though they are in the beta reality, while potentially acknowledging that the training mission is a simulation. This is not disimilar to training undergone for actual space missions, where astronauts will be placed in simulators, and put through various scenarios.