Gatekeeper (noun)

1. A person who is responsible for guarding or controlling access to a place.
2. A person or organization that regulates or controls the access or flow of something.
3. A person who has the power to decide who is allowed to enter or pass through a place.

Origin:

Late middle english: from gate + -keeper.

Examples:

1. The gatekeeper at the castle entrance checked their passports before allowing them in.
2. The gatekeeper of the publishing industry decides which books are accepted and published.
3. The gatekeeper at the festival entrance checked tickets and let people in.
4. The gatekeeper of the internet controls the flow of information and decides which websites are accessible.
5. The security guard at the entrance was acting as the gatekeeper, checking ID cards before allowing employees into the building.