Gilbert was optioned to Triple-A Reno on Thursday.
Gilbert's first major-league appearance this year came as a starter, but he tossed three innings out of the bullpen during Monday's loss to the Dodgers. He'll now head back to the minors, where he's served as a starter in each of his first two outings of the season and will pitch as a starter upon his return. Luis Frias and Tyler Holton were called up to provide additional bullpen depth.