Gunston was raised in Melbourne and attended Haileybury College, playing school football alongside three other future AFL players (Tom Scully, Jack Hutchins, and Tom Lynch). His father, Ray Gunston, played VFA football for Brunswick, and later moved into sports administration. Gunston began his junior career with the Beaumaris Football Club. He originally played mainly as a midfielder, but after growing 15 centimetres (5.9 in) in two years he began to play more in key positions. In 2009, the year he became eligible for the AFL Draft, Gunston was selected for the Sandringham Dragons, a TAC Cup team. Early in the season, he dislocated his elbow in a practice game, an injury which took eight weeks to heal. Gunston played for the Dragons in only their last six matches, but was nonetheless a second-round pick in the 2009 AFL National Draft, taken by Adelaide with the 29th pick overall.