On 31 January and 1 February, Gagai played for the Knights in the 2015 NRL Auckland Nines. On 13 February, he played for the Indigenous All Stars against the NRL All Stars in the annual All Stars match, playing off the interchange bench for the at Cbus Super Stadium. The Indigenous side won 20-6 over the NRL All Stars. On 10 April, he re-signed with the Knights on a 2-year contract after strong interest from his former club Brisbane Broncos. On 8 July, he made his State of Origin debut on the wing for Queensland against New South Wales in Game 3 of the 2015 State of Origin series. He scored a try on debut, which was the first try of the game, in Queensland's 52-6 win. He finished the 2015 season having played in all of the Knights 24 matches, scoring 7 tries and kicking 2 goals as the club finished last. On 26 September, he played for the Prime Minister's XIII against Papua New Guinea, playing at centre in his team's 40-12 win in Port Moresby. On 15 December, he was named on the wing for the Indigenous All Stars to play against the World All Stars on 13 February 2016.