After retiring as a player, Gilmer became an assistant coach with the Pittsburgh Steelers under Parker in 1957. After four years there and four with the Minnesota Vikings under Norm Van Brocklin, he succeeded George Wilson as Lions head coach in January 1965.
Following his retirement from playing, Gilmer took on a coaching role with the Pittsburgh Steelers under Parker in 1957. He then spent four years with the Minnesota Vikings under Van Brocklin before becoming head coach of the Detroit Lions after George Wilson's departure in January 1965.