A gateway system is a system in which the members of the system have the experience of existing in another world. The world(s) in question may have a certain reality to them, and be very lifelike. These worlds may be considered metaphysically (or otherwise) real to some systems, or experienced as subjectively real but not metaphysically real to others. Some may consider their worlds to be entirely neurally generated or consciously created, while others may view their worlds as being a real place as any other, just with a connection to a body that everyone in the system can use.
Gateway systems may also have large amounts of members which come in and out of the system (which may or may not be walk-ins), or large populations due to their unique setup. Sometimes the gateway in question may experience their world(s) as being the source of the large population (i.e. receiving systemmates from other places), while others may consider some or all of their members to be ingrained in the system.