"It was one of the toughest games I've ever been involved in. Great grit from the boys to stick in there in the end. Really proud of them," Hawke's Bay captain Tom Parsons told Sky Sport after the game.
"Shield footy is always like that. A massive contest and you've got to put everything into it otherwise you lose it."
Harbour edged ahead several times in the clash as the Bay's Shield defence looked increasingly like it would be unlucky number 13.
In a superb 70-metre move, starting with Shaun Stevenson and ending with Li finishing his third try, Harbour looked like they landed a big blow to the Bay's hopes to go ahead 23-15 with 20 minutes to go.
But the home side refused to let the challengers breathe as their patience and persistence paid off, ultimately completing a thrilling comeback.
"If we put phases together and put the pressure on, we knew we would come up with the result in the end. But credit to Harbour, they really pushed us and put us under pressure there at times," Parsons said.
"The boys love it; they thrive off it," he added about the long-running defence. "It gets us up at home and it's something special for the province. We love getting the crowds that we get here so we've got to play for that."
A disappointed Gatland said his side gave it their all.
"It's a pretty tough pill to swallow. These opportunities don't come around often and we really believed that we could turn up today and get it done. I think we left a lot out there, especially in the first half having the wind behind us and left a few points out there. We fought all the way to the end but credit to Hawke's Bay – that's why they've defended it for so long.
"We knew coming down here, they weren't going to just give it to you and you're not going to get any favours. You have to go out there and earn it and we didn't quite do enough today."
It was also a successful return to rugby for All Blacks lock Brodie Retallick, who was solid for Hawke's Bay in his first NPC game since 2012, after being sidelined with a broken cheekbone for six weeks as he aims to return to the national side for next weekend's clash against Argentina in Hamilton.
Hawke's Bay 25 (Chase Tiatia, Lolagi Visinia, Kianu Kereru-Symes tries; Lincoln McClutchie 2 cons, 2 pens)
North Harbour 23 (Tevita Li 3 tries; Bryn Gatland con, 2 pens)
Halftime: 12-15