In 1959, Gasnier moved up from reserve to Test team, making his international debut for Australia against New Zealand in the 1st Test of 1959 in Sydney. He played in all three Tests of that series, and went on to tour Britain with the Kangaroos. On the tour, he scored a hat trick in the 1st Test 22–14 victory against Great Britain at Swinton's Station Road (attendance 35,224). However the Ashes were retained by the home nation after wins in the remaining Tests at Headingley Rugby Stadium, Leeds (11–10, attendance 30,184) and Central Park (Wigan) (18–12, attendance 26,089). In 1960, he played in World Cup finals as a member of the national team. He is listed on the "Australian Players Register" as Kangaroo No. 339.