In round 13 of the 2014 NRL season, Gagan made his NRL debut for Cronulla-Sutherland against the St. George Illawarra Dragons, playing on the wing in the Sharks' 0–30 loss at WIN Stadium.[10][11] In July, he re-signed with Cronulla on a two-year contract. Gagan made a total of seven appearances for Cronulla in the 2014 NRL season as the club finished last on the table.[12][13]
2016
In November, after missing out on NRL selection for Cronulla-Sutherland throughout the entire 2015 and 2016 seasons, Gagan signed a one-year contract with the Newcastle Knights starting in 2017.[14]
2017
Gagan made his Knights debut in round 5 of the 2017 season against his former club Cronulla-Sutherland, scoring a try.[15][16] That would be his lone appearance for the Knights as he was not offered a new contract beyond 2017.[17] In November, he signed a 1-year contract with the South Sydney Rabbitohs starting in 2018.[18]
Gagan made his debut for South Sydney in round 3 of the 2019 NRL season against Manly-Warringah after being called into the side as a replacement for the injured Greg Inglis.[20]
2020
In February, it was revealed that Gagan had been released by South Sydney.[21]
In 2021, Gagan joined Queensland Cup side Norths. The Norths Devils would go on to win the Queensland Cup Grand Final in both 2021 and 2022. Gagan was named as Man of the Match and the Duncan Hall Medal winner in the 2022 Queensland Cup Grand Final. In late 2022, Gagan signed a contract to join one of his former teams in Newtown ahead of the 2023 NSW Cup season.[23]
2023
On 17 August, Gagan re-joined the Norths Devils after leaving Newtown earlier in the year on compassionate grounds.[24]
