"Gateway" is a term invented and used by the greater plural community to describe entryways, holes, doors, or gates in and out of a psyche. It is usually viewed as a spiritual concept, and is indeed part of some very old traditions. And gateways may be open or shut, and sometimes may be guarded by a gatekeeper.
People or systems with gateways perceive there to be a world, universe, or reality beyond their gateway. Some people believe or have experienced that they can leave through their gateway and enter someone else's psyche if they have a compatible gateway, too.
We, the Inmara, have two gateways, The Hole in the Sky and The Abyss. We do not yet know much about them, other than that we perceive them and that some of us remember coming through them. We expect, if they are not merely psychological constructs, that the space beyond them is a substrate of the rest of the universe that can support a form of consciousness, and through which entities can travel. We have started Project Open Door in an attempt to explore this further.