Pc Gillian Weatherley has been dismissed from the Metropolitan Police without notice after she was found guilty of gross misconduct.
She was on duty in Downing Street on the day of the confrontation between then-chief whip Andrew Mitchell and fellow constable Toby Rowland in September 2012.
The next day, Weatherley sent a photograph of an email that Rowland had sent to his bosses about the row to another officer, James Glanville. He was later sacked for leaking the information to the Sun newspaper.
The force said she had given "inaccurate and misleading" statements to officers investigating the aftermath of the row, and had been suspended from duty since her arrest in February last year. Prosecutors have already decided not to press charges.