A press Q&A session is called a "gaggle" because it is an informal, often noisy gathering of reporters that mimics a "gaggle of geese". It is a term used specifically to describe an informal, off-camera Q&A session with the White House press secretary or other officials.

Here is why it is called a gaggle:

Informality: Unlike the structured, televised daily press briefing, a gaggle is casual, often occurring in hallways, on Air Force One, or in the press secretary's office.