Eight years since last wearing Australia's green and gold, and nine since sharing in the 2017 Kangaroos World Cup victory, the 35-year-old has joined the Kiwis on their quest for glory.
"(2017 World Cup victory) was an awesome experience," Gagai said.
"It was nine years ago now so it was a long time ago. Going up against all the other countries, you don't want to let your team down, you don't want to let your country down.
"It's no different this time around. I'll be doing everything I can to get a spot on that team and do everything I can to bring back a World Cup for New Zealand."
The Queensland State of Origin representative is eligible for New Zealand through his mother, but never thought it would be possible as previous international eligibility laws excluded Origin players from the English or New Zealand national teams.
The ARLC overhauled Origin eligibility rules in February, allowing players to represent any nation and also remain eligible for NSW or Queensland.
For Gagai, it was the conversation telling his Mum that triggered the emotional weight of this decision to really sink in.
"To be able to represent her, and (her journey) moving across to Australia to create a home for me and my family, that was probably one of the most proudest moments," he said.
"But being Kiwi for me isn't just where you're born and raised, it's who you are.
"My family are from New Zealand, so it's just a part of me and my culture."
The 340-game NRL veteran proudly takes every opportunity to represent all of his culture, having previously lined up for both the Indigenous and Maori All Stars, alongside his Australian caps.
"I'm obviously still yet to take the field for the Kiwis, but I've represented Australia, Queensland, Maori All-Stars, Indigenous All-Stars, and then this was the last thing for me to do," Gagai said.
The move came together quickly, with the new Kiwi only receiving a phone call a couple of weeks ago from assistant coach Adam Blair to ask about his interest in representing New Zealand.
The World Cup will cap off a whirlwind end to the NRL season as Gagai was instrumental in Newcastle's charge to the grand final, and their heartbreaking one-point loss.
With a blockbuster first game against the Kangaroos in Sydney coming up on Thursday night, the centre could be set for another fierce match-up against Latrell Mitchell.
It comes just weeks after a fine individual performance as Gagai's Newcastle eliminated Mitchell's South Sydney from the finals.
"He (Mitchell) has got plenty over me before, so he's just a competitive footballer, as am I, and we just want to do everything we can for our teams to win," the 35-year-old said.
