Gallman entered his sophomore year in 2015 as the starter. As a redshirt sophomore in 2015, Gallman played in 14 games. In the season opener against Wofford, he had 92 rushing yards and two rushing touchdowns. On Halloween, against North Carolina State, he had 172 rushing yards and a rushing touchdown. In the ACC Conference Championship against North Carolina, he had 187 rushing yards, one rushing touchdown, and four receptions for 68 receiving yards and a receiving touchdown as part of a strong performance. Clemson's successful season qualified them for the College Football Playoff. In the Orange Bowl against Oklahoma in the National Semifinals, he had 150 rushing yards and two rushing touchdowns in the victory. In the 2016 College Football Playoff National Championship against Alabama, he had 45 rushing yards and a rushing touchdown to go along with three receptions for 61 yards in the 45–40 loss. Overall, he finished the season with 1,514 rushing yards and 13 touchdowns.