In 1983, Gunn travelled with the Scotland Under-19s to Mexico for the 1983 FIFA World Youth Championship. Gunn played in all four of Scotland's matches and it was a formative experience, as he faced a variety of challenges in the tournament. The adidas ball in use "really zipped through the air", which encouraged long-range shots at the high altitude. In the first match, against South Korea, an element of the 26,000 crowd "clearly didn't like us at all ... I got absolutely pelted with coins ... and rotten fruit". The final insult was when he was hit on the back of the neck by a rancid-smelling plastic bag containing rotten fish. Gunn found it "intimidation ... a nightmare" but adopted the strategy of patrolling the edge of his area to stay out of range and had "one of the best games of my life": he kept a clean sheet as Scotland won 2–0. Scotland's coach and future manager of the full international team Andy Roxburgh told Gunn "it was the best performance he'd ever seen from a Scottish goalkeeper, at any level." The final group match, which Scotland needed to win in order to progress, was against the home side, Mexico. The official attendance at the Azteca Stadium was 86,582, although Gunn believes the true figure was over 100,000. Once more, Gunn was targeted with a variety of missiles, including a bottle of Johnnie Walker, but as Scotland held on to their early 1–0 lead, the fans became disillusioned with their own team and switched to sending a "hail of bottles and coins" on the Mexican players. Scotland lost 1–0 in the quarter final against Poland.