The Reds selected Gullett in the first round of the 1969 Major League Baseball draft. He pitched for the Sioux Falls Packers of the Northern League that season. In 1970, Gullett was so impressive in spring training, despite his inexperience, he made the big league roster of a team that would go on to win the NL Pennant. Pitching in relief of starter Ray Washburn, Gullett debuted on April 10, 1970, on the road against the San Francisco Giants. Gullett had an outstanding rookie season, appearing in 44 games (42 in relief) posting a 5-2 record and a 2.43 earned run average. In the 1970 World Series against the Baltimore Orioles, Gullett pitched innings and allowed just one earned run (1.35 earned run average) as he and veteran Clay Carroll helped keep an injury-riddled pitching staff competitive in the series.