GU1 is a world specialising in high tech, high performance military and mercenary exercises. The highest level of training is held here, as are the testing labs, immortalisation, and control room. Much of the world has been stripped of resources, and has left it barren and rotting.
GU4 is a training base for millions of soldiers, assassins and mercenaries. Exams for multiversal mercenary positions are held here, and those who pass are sent to GU1 for further training.
Much like GU1 and a lot of the worlds in the early parts of the G branch,the world has been stripped of anything useful. Nature has pushed back, however, and has taken over old buildings in places. It's still a rather barren world, but it has the ability to regrow.