In the chamber of Gemariah - He was one of the princes of Judah. See Jeremiah 36:12.
Gemariah seems to have inherited his father‘s office of public scribe or secretary of state (see 2 Kings 22:3). As brother of Ahikam, he would be favorable to Jeremiah.
The higher court - The inner court; into which it was not lawful for the people to enter, but the chamber probably itself formed one of its sides, and could be approached from the outer court.