Michael Carayannis on The Daily Telegraph is reporting the contract will be a three-year deal, which is what the star centre was chasing.
It's understood the Eels were also in the mix for Gagai, while the Broncos had been weighing up an offer.
The Rabbitohs wanted to make Gagai a priority re-signing, but were unprepared to offer any more than a one-year deal for the 30-year-old, in a similar way to their treatment of long-term star half Adam Reynolds.
Reynolds will now depart for the Broncos.
The move of Gagai to the Knights, where he played between 2012 and 2017, spending the club's toughest period in the Hunter, could set the wheels in motions in other areas of the transfer market.
It has been reported for some time now that if the Rabbitohs couldn't land Gagai, they would chase off-contract former million dollar man Anthony Milford.
The Broncos are understood to be unlikely to offer Milford a new contract, but with Menji Marshall undecided on his future, incoming coach Jason Demetriou may reach out to Milford as a replacement on the bench for the men from Redfern.
Gagai has been one of the most consistent performers at Origin level for Queensland, scoring 11 tries in 19 appearances and has won numerous player of the match awards at the representative level.
His club form has been far improved this year compared to previous seasons, with Gagai part of the Rabbitohs' dangerous left edge.
With eight tries to his name for the season, he has also benefited from having Alex Johnston playing outside him.
All Run Metres
The contract in Newcastle could well be the 30-year-old's last, with Gagai joining a Newcastle backline with plenty of depth.
It's unclear at this stage where he would slot in, given the utility value of Kurt Mann, and the centre pairing of Enari Tuala and Bradman Best, while outside backs Dominic Young, Edrick Lee, Hymel Hunt, Gehamat Shibasaki and young gun Starford To'a are also all at the club.