He has played for the Indigenous All Stars, Queensland in the State of Origin series and the Prime Minister's XIII.

[9][10] In round 1, Gagai made his NRL debut for the Brisbane Broncos against the North Queensland Cowboys on the wing, scoring a try in the 16–14 loss at Suncorp Stadium.

[18] On 28 January, Gagai was banned from the Queensland Origin camp by Mal Meninga after a discipline breach.

[24] On 10 April, he re-signed with Newcastle on a two-year contract after strong interest from his former club Brisbane Broncos.

[33] In round 10 against the Cronulla-Sutherland Sharks, Gagai played his 100th NRL career game in the Knights' 0–62 loss at Hunter Stadium.

[41][42] Gagai made his third consecutive appearance for the Indigenous All Stars, starting on the wing and scoring a try in the 34–8 win.

[44][45] On 12 July, Gagai won the Wally Lewis Medal as the player of the 2017 State of Origin series.

[47] On 27 October, Gagai made his international debut for Australia against England, playing on the wing in the 18–4 win at AAMI Park.

[49] Gagai played in 5 matches and scored 3 tries in the tournament including starting on the wing in the Kangaroos gritty 6-0 World Cup final victory over England at Suncorp Stadium.

During the 2018 All-Stars match at AAMI Park, Melbourne, Gagai was eligible to play for both representative teams as he had Torres Strait Island and Maori heritage.

[54][55] Before the 2019 NRL season started Gagai chose to play for New Zealand Maori kiwis against the Indigenous Australian All Stars and in that game he scored two tries.

In Game 1 of the 2019 State of Origin series, Gagai scored 2 tries and won man of the match in Queensland's 18–14 victory at Suncorp Stadium.

[56] In round 19 against St. George, Gagai scored two tries as South Sydney won the match 20-16 after the final siren had sounded.

[58] In round 5 of the 2020 NRL season, Gagai scored two tries as South Sydney defeated the Gold Coast 32–12 at Bankwest Stadium.

[59] In round 9, Gagai scored his first NRL hat trick as Souths defeated the West Tigers 18–10 at Bankwest Stadium.

He played in the club's preliminary final defeat against Penrith as Souths fell at that stage in the competition for a third year in a row.

[64] In Game 3 of the 2022 State of Origin series, Gagai was sent to the sin bin for punching New South Wales player Matt Burton during Queensland's 22-12 victory.

[65] In round 19 of the 2022 NRL season, Gagai was sent to the sin bin for dissent during Newcastle's 42-12 loss against the Sydney Roosters.