At some airports there is a large room, usually with loads of seating, dedicated to a single gate. Staff check boarding cards and others documents on entry to the room but you can usually then go no further, initially at least, so you wait there until the aircraft is ready for boarding - at which point the doors at the exit end of the room which lead to the jetty proper are opened and you can head on board.
Used to be common at Gatwick, not sure about now and not so much so at Heathrow these days.