The Mariners announced the moves Thursday and have 19 non-roster invitees coming to their major league camp.
The 33-year-old Gerut broke into the majors in Cleveland under new Seattle manager Eric Wedge. He played 32 games last season with Milwaukee but was sidelined for more than two months with a bruised left heel.
The 33-year-old Robertson had most of his success with Detroit, winning 12 games in 2004 and 13 games in 2006. Last season, Robertson was 6-8 with a 5.47 ERA in 19 games with Florida.