A gateway system can refer to one of two things: A system that believes their headspace to be a real dimension or place somewhere, or a system in which many or all of the members originate from another world, using a human body to interact with this world and often coming and going as they please. These systems are typically metaphysical in nature. 

With the first definition, a gateway system may feel for various reasons that their headspace (or otherworld) is a real place due to various aspects of the space or their experiences inside of it. Some have headspaces that are functioning planets or even universes, or appear to react as an independent world would, with temperature, climate, animals, civilizations, historical in-system events and more that are out of anyone's control. They may have metaphysical beliefs (like their headspace existing on another spiritual plane) that explain this further.

With the second definition, there are those who experience frequent walk-ins or who have large amounts of members that seem to stem from non-psychological origins. The member(s) origins may be from elsewhere, such as from an external spiritual world, and they may be prone to other metaphysical experiences or have metaphysical traits and abilities associated with their origin. In rare instances the first meaning of gateway system connects to the second, and people may view their headmates as being metaphysical because they come from their headspace.