Gatekeepers are heralds sworn to uphold the natural order of the cosmos. They are watcher's in the shadows, ever vigilant for incursions by beings from other alien planes. To a Gatekeeper, it matters not if you are a fiend, celestial, or elemental, good, evil, or unaligned - if you are not of this world, you are unwelcome.
All gatekeepers follow a similar set of tenets.
Close Shut the Outer Gates. The realms beyond this world are alien and dangerous. They must be kept separate and apart, and the natural order of the cosmos maintained.
Unwelcome Spirits. Beings from distant planes are as dangerous and alien as the realms they hail from. They must never trusted, and extinguished or driven back wherever they are found.
Trust in What is Real. The physical world is the only truly real one. Worship the gods if you will, but let the gods stay in their heavens. You live in the world. Do not forget that.
Exorcism. Everyone’s body and mind belongs to themself. Do not tolerate possessions, and end such corruption wherever it is found. Even a willing possession is an abomination.
Add the planar and negation spell schools to your list of herald spells.
At 3rd level you gain the following channel divinity options:
At 7th level your divine smites deal an additional 1d8 damage against any creature turned by your Turn the Outsider ability. Whenever you kill or banish such a creature, you may regain a use of your divine smite. Once you have regained a number of divine smites in this fashion equal to your Charisma modifier you may not do so again until you finish a long rest.
At 15th level you gain an expertise die on all Intelligence, Wisdom, and Charisma saving throws. When you succeed on such a saving throw, you may use your reaction to deal damage equal to your divine smite to the creature that forced you to make the saving throw. Once you have done so you cannot do so again until you finish a short or long rest.
At 20th level your focus and dedication make you a living bastion against invasive spirits. You gain Truesight. Whenever you critically hit or smite a creature turned by your Turn the Outsider ability you may force them to make a Charisma saving throw against your spell save DC or be banished back to their home plane if they are not already there. On a successful saving throw, a creature is immune to this ability for 24 hours.