Gatekeepers are liminal -- persons on the thresholds -- between one subsystem and another in a multiple's system. Generally they control access from one subsystem to another, which can include knowledge, memories, communication, feelings, internal landscape contact, etc.
Not all subsystems must have a gatekeeper who is a solid entity. Gatekeepers can be portals or interfaces in the internal landscape from one area of the landscape to another. Literally, one can have portals in the mind between two drastically different landscapes, or separate rooms in an internal house, or perhaps one or two people have the ability to simply "BAMPH" to another place/time in the internal landscape...but no one else can follow them there.
Gatekeepers can be people, fragments, veils, landscape constructs, etc.
One of the fun parts of Gatekeepers is that they often blend in. One system will think the Gatekeeper is a natural part of what's going on, and not suspect that there's anything on the other side of the Gatekeeper. So someone has a headcount of 5 people...Joe is one of those people. Joe interacts with them just like anyone else. Little do any of them know -- possibly including Joe -- that Joe has another 5 people behind *his* eyes -- a multiple within a multiple. He's a Gatekeeper to a subsystem... possibly with their OWN internal landscape, fragments, constructs, and also possibly their own gatekeepers.
Apparently the above is a bit too vague, I will give it another try. This is my understanding of some of the many ways gatekeepers can come to exist. This is an oversimplification using an example that clearly demonstrates motivation to create subsystems and gatekeepers.
Many multiple systems evolve because of betrayal. Whether it is betrayal by outside beings or internal beings. When an abusive situation develops, at first a person is too naieve to recognize that they are being taken advantage of. Sometimes there are pleasant aspects to the abuse, or there is a great deal of fear for what will happen if you resist. And there is anger. All of these conflicting emotions boil down to compliance vs resistance. I do not know whether many who openly resist make it to the point of multiplicity. Or perhaps resistance only results in escalation of threats or domination/manipulation further exacerbating the plausible abuse situation.
When the situation is recurring, this division becomes rudimentary splitting and becomes an inner war between the need to capitulate and the need to defy. The defiant aspects will plot and plan how to overthrow the agressor and lock down a method for how to succeed and when the signals are recieved that abuse is immanent again a switch takes place and those who normally comply simply submit again, sparing the others of unknowable horrors that might happen in the inevitable escalations. The agressors are too powerful and the best course of action is to comply so as not to anger them too greatly. The defiant aspects end up being betrayed by other aspects in the body whose job it is to make the best of the situation, soften the impact as much as possible.
Self-betrayal leads to distrust of oneself. The fact that there are no internal divisions means that when the defiant aspects plot, the compliant aspects know everything. For these defiant aspects to meet their need for independence, autonomy, and an end to the abuse, they need to be able to plot in privacy. However they cannot fully close themselves off from everything going on externally or the thoughts of the compliant group. They create a permeable barrier, a filter if you will, between themselves and the others. If they do defy they need to keep the sensory input from the compliant group, who will -- out of fear -- take control and undo all the resistance the defiant group can muster.
Meantime the compliant group is probably all too aware that the acts they suffer are distressing others in the system. They need to take sensory input but do not want to share it with the others, especially the ones who need to run the daily life or the defiant ones who threaten the status quo and threaten to bring on the unknowable and possible escalation of the threats and pain. They also need to create barriers that are permeable and to steal sensory input and blind others to the events that are going on.
None of these internal aspects can afford to simply disappear or to close themselves off -- they need a flexible method of montioring what is going on, taking control of the body, stealing sensory input, and blending in so no one suspects what they are up to in the recesses of the mind.
A gatekeeper is usually an aspect or construct behind which a related group of entities hides. The gatekeeper can act on their behalf -- sometimes as a veil or mask, sometimes as an internal entity in its own right but with motivations and ideas leaking from the enclosed subsystem. The enclosed entities can glean sensory input from the gatekeeper. They can even force a switch of which subsystem is fronting and this probably accounts for time loss better than simply switching who is fronting when the entities are in the same subsystem.
The most notable gatekeeper in the popular novels on multiples is generally the unsuspecting front who handles the normal chores of living. They do not know they are a gatekeeper to the inner system, but they are influenced by those they enclose.
I can't say if every system has gatekeepers or subsystems -- however it is a very advanced idea and usually is not uncovered early on in poking around and introducing yourself to your selves. Because of the possibility of self-betrayal -- or keeping an ace up one's sleeve -- some entities and subsystems will not reveal themselves for a long while if at all -- and the compliant group may still fear the impact of the things they suffered on the rest of the group, so they may not wish to come forward.
Also, knowing about the subsystems does not necessarily unmake them. There are other good reasons for subsystems than what I describe -- this is only one of the more clear reasons they may evolve and be elusive. Hopefully we will become inspired to write up more, or others may add to this explanation.