Running back Frank Gore is signing a one-day contract with San Francisco to retire a 49er, capping off a 16-year NFL career at one of the most punishing positions in football.BREAKING: Frank Gore will retire with in ceremonial signing Thursday via Cam Inman (@CamInman) Gore, who has recently transitioned to boxing, entered the league at 22 years old and is retiring at 39 after . He spent the...