Knight of the Gate is the title given to the commander of the Bloody Gate, the castle that safeguards the eastern roadway into the Vale of Arryn. Traditionally, he asks the question, "Who would pass the Bloody Gate?" to all who would pass through.
The current Knight of the Gate is Ser Donnel Waynwood.
In the books
In the A Song of Ice and Fire novels, the Knight of the Gate is a knight chosen by the Lord of the Eyrie as commander of the Bloody Gate's garrison and not an inherited post.