"What would you want if it was your father [coaching the Lions]?" he asks. "It's not that I want the All Blacks to lose, it's that I want Dad to do well. It will be amazing if the Lions can do well against the best side in the world. It will be a very proud moment for my family if Dad can achieve that goal."
The landscape of modern rugby is littered with dynasties - from the Fords and Farrells of England, the Youngs of Wales and the Barrett clan of New Zealand - but never before have a father and son been pitted against each other in a Lions series.
The pair have been joking about the possibility since last autumn, when Bryn helped North Harbour win promotion to New Zealand's Premiership. Back then, all the chatter was of Junior being "targeted", a suggestion which prompted Bryn to tell his dad: "Don't worry, I've got a few tricks up my sleeve."
Now, the tone is more diplomatic, with Bryn clearly aware of the burden of expectation being shouldered by his father. He was almost there to see Warren lead the Lions to victory in the test series in Australia four years ago, but a delayed flight left him in mid-air while the match was on. When he arrived in Sydney his phone was filled with messages of congratulations after the emphatic 41-16 win.
Tomorrow, the Gatlands will all have front row seats, with Warren's wife, Trudi, having bought 30 tickets to the opening game before her son had even been named in the Barbarians squad.
Despite their father spending much of his year in Britain, the Gatlands remain ferociously tight-knit.
In an interview last year, Trudi revealed how the first thing Warren did on returning to his home country with Wales for their three-test summer tour last year was secure a ticket to watch Bryn play for North Harbour against Northland.
"I often compare it to husbands and wives in the forces, when one of the couple is away for long stretches," she reflected. "There are also people who commute or have highly powered careers who might not see each other a lot on a daily basis. In a lot of ways we are very lucky and the times we have are good."
Tomorrow might be an unusual family reunion, with little time for pre-match pleasantries.
Bryn knows a stand-out performance could propel him into the public's imagination, and serve as a reminder to the coaching staff at Blues, the Super Rugby franchise for whom he recently provided injury cover, of his many talents.
He is certainly not being underestimated by Gatland's squad. Greig Laidlaw admitted they had been "doing our homework" on his game, while Sam Warburton is already familiar with his gifts.
"I've known Bryn now for years," he said. "He was coming in to kick with the Wales squad six, seven years ago. He has shown a lot of courage in the past and will definitely do the same again now. It's a great achievement for him to be in this position."
The chance of Bryn and his fellow Barbarians - who are largely semi-professionals - putting one over his old man is slim. For now, just making him proud will be enough.
