Getter Valhalla is a fan-named pocket universe where Getter robots and pilots from across the multiverse converge in a never-ending battle to continuously evolve and grow stronger with the pinnacle of evolution appearing to be the Getter Emperor.
The notion of Getter Valhalla first appeared at the end of Getter Robo Armageddon where the pilots of the Shin Getter Robo are transported when the Emperor had appeared. Subsequent appearances would depict alternate worlds where humanity had actually combined themselves with Getter robots to escape worlds polluted by Getter Rays and get the necessary power to break through atmospheres and fight in space. The Getter Emperor would often be seen as an observer and as something the other Getter pilots strive to obtain.
As a result of the influx of Getter Rays in this reality, the memories and powers of the Getter robots gather from nearly every corner of the multiverse to find a way to perfect evolution. Because of the living conditions, pilots cannot survive outside of a Getter robot for very long and to get necessary parts they often have to fight other pilots to the death to obtain the equipment. This survival-of-the-fittest way of thinking however is ridiculed by people including several versions of Ryoma Nagare. In fact, the Getter Emperor seems to want these individuals to fight against the established status quo brought on by the harsh reality watching as these fighters make sure the Emperor never rises.