While Gloucester head coach Bryan Redpath will obviously be disappointed with the defeat, especially since they were also beaten in last year's final, his side continued their post-Christmas resurgence with a grandstand display.
Both teams were without several international stars because of Saturday's Six Nations fixtures, with Northampton missing Ben Foden, Chris Ashton, Dylan Hartley and Euan Murray and Gloucester without Mike Tindall, Gareth Delve and Marco Bortolami.
Gloucester came into the game as underdogs but it was Redpath's side who made the brighter start, with Lesley Vainikolo looking for work and making plenty of carries.
Despite the open nature of the game it was only 9-6 to Gloucester after 23 minutes, with Robinson slotting three penalties to Stephen Myler's two.
But a match that had merely been smouldering then took flame, Gloucester's Qera providing the spark when he crashed over from close range following a break from Robinson.
Saints hit back immediately, Shane Geraghty, playing at full-back, counter-attacking from deep and initiating a sweeping move which ended with loose-head prop Tonga'uiha burrowing over.
Myler kicked the extras to reduce the deficit to one point and Jim Mallinder's side then started to pour on the pressure as the game began to break up even further.
First Phil Dowson went on a barrelling run and then, in a passage of play that epitomised the free-flowing nature of the game, James Simpson-Daniel broke from deep for Gloucester before Saints lock Courtney Lawes stole the ball and fed Bruce Reihana.
The Kiwi passed inside to hooker Brett Sharman and the ball was eventually shipped right for centre Downey to cut inside, leave the Gloucester defence flat-footed and touch down next to the posts.
Myler's second successful conversion put Northampton 20-14 in front at the break and Gloucester reshuffled their back division for the second half, with Simpson-Daniel moving into midfield instead of Tim Molenaar and Charlie Sharples slotting in on the wing.
But it was up front where the Cherry and Whites did the damage shortly after the restart, referee Alan Lewis awarding a penalty try when Saints' front-row disintegrated three metres out from their try-line.
Robinson's conversion edged Gloucester two points ahead, but as in the first half, they held their lead for barely three minutes.
Northampton stormed upfield from the restart and Tonga'uiha turned provider, slipping Dickson the ball and the scrum-half scampering over for a try.
Myler slotted the extras to make it 27-21, before Gloucester thought they had wiped out their deficit approaching the hour-mark.
Robinson weaved his way through Northampton's defence and thought he had grounded the ball on Saints' line, but television match official Ed Morrison ruled otherwise.
There was no let-up in the pace, however, with man-of-the-match Tonga'uiha intercepting in his own 22 and rampaging 60 yards before shipping the ball inside.
The ball went left, but Northampton back-row Apo Satala put in a magnificent cover tackle to deny Brian Mujati a try in the corner.
Myler, who finished with a 100% goal-kicking record and 15 points, and Robinson, who slotted 14 points, exchanged further penalties as the clock ticked down.
But the six-point deficit ultimately proved too much for Gloucester as Northampton held out to clinch a contender for game of the season.
Northampton director of rugby Jim Mallinder: "Cup finals are all about winning, you would take 3-0 in a final.
"But it is more encouraging when you can play like that. There were two teams who wanted to play rugby, it was a good advert for the game.
"It shows how far the side has progressed, and it was great for our fans. It was special."
Gloucester head coach Bryan Redpath: "It is never easy, losing finals. It was a game that could have gone either way.
"Nicky Robinson said he'd scored in the second half (television replays suggested otherwise), but that's life.
"We didn't control possession well enough and there were one or two little errors, but credit to Northampton."
Northampton: Geraghty, Reihana, Clarke, Downey, Diggin, Myler, Dickson, Tonga'uiha, Sharman, Mujati, Lawes, Kruger, Dowson, Best, Wilson. Replacements: Ansbro for Clarke (75), Long for Sharman (62), Easter for Lawes (70).
Not Used: Dreyer, Vickers, Hopley, Dickens, Everitt.
Gloucester: Burns, Simpson-Daniel, Molenaar, Fuimaono-Sapolu, Vainikolo, Robinson, R. Lawson, Wood, Azam, Capdevielle, James, Brown, Buxton, Qera, Eustace. Replacements: Voyce for Burns (63), Sharples for Molenaar (41), Lewis for R. Lawson (69), Dawidiuk for Azam (75), Satala for Brown (37), Hazell for Qera (69).
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