According to the list of registered candidates that the city has posted online, Gardner filed his papers on Thursday.
Gardner previously served as the chair of the Toronto Police Services Board.
He is the second former councillor to sign up to run against Mayor Rob Ford.
David Soknacki, who has also served on council, registered for the election earlier this month.
Ford, who is seeking a second term as mayor, registered on the first day that it was possible to do so. He has predicted that the election campaign is going to be "a bloodbath."