An informal briefing by the White House press secretary that is on the record but video recording is not allowed. It can occur anywhere, such as on Air Force One, but it often describes the informal interactions between the press and the press secretary that occur before a formal White House briefing.
The term likens the members of the press corps to a “gaggle of geese” honking.
Washington Monthly: “Gaggles historically refer to informal briefings the press secretary conducts with the press pool rather than the entire press corps. They used to happen in the morning, they were more or less off the record, and their purpose was mostly to exchange information – the president’s schedule and briefing schedule, from the administration side; heads-up on likely topics or early comment on pressing issues, from the news side. Briefings were what everybody knows them to be.”