After some incessant early pressure from the visitors, Saints prop Emmanuel Iyogan picked up a yellow card for pulling back an opponent at a scrum inside six minutes.
Deian Gwynne barrelled over following a maul from a close-range line-out seconds later, but Charlie Atkinson’s subsequent kick from the whitewash struck the upright.
Gloucester added a second score just before Iyogan returned as they spread the play from left to right for Ollie Thorley to race over in space, with Atkinson adding a fine conversion from out wide for a 12-0 lead.
Saints reduced the arrears on 20 minutes when Gloucester turned it over in their own half and George Hendy received a quick pass to blaze into space down the left and plunge over, with Fin Smith slotting the extras.
The scores were tied seconds after the restart when Saints raced the length of the field from deep in their own 22, with Furbank bursting away on the right before Tommy Freeman found Fin Smith to finish the move by the posts. Smith added the conversion himself to give Saints a two-point lead.
It did not last long as Gloucester replied with a stunning score of their own as Caolan Englefield worked some room with a couple of dummies to send Seb Atkinson over in the left corner, with Charlie Atkinson’s boot making it 19-14 inside 28 minutes.
Six minutes later centre Rory Hutchinson latched on to Fin Smith’s close-range grubber kick to mark his 200th appearance for the club with a try, though Smith’s routine conversion was missed leaving the scores level.
After the break Gloucester picked up the bonus point try as Ben Loader was left in space on the left to dot down after Seb Atkinson had worked some space, though Charlie Atkinson’s conversion attempt drifted wide.
It took Saints seconds to follow suit and make it 24-24 as Freeman dotted down in the right corner for his 16th Prem try after some slick hands following Tom Pearson’s break through some slack Gloucester defence.
Saints went to sleep as Gloucester added their fifth try just before the hour. Seb Atkinson popped the ball to Max Llewellyn and he darted through untouched, though Charlie Atkinson’s subsequent kick drifted wide.
Saints roared back and Sam Graham barrelled over by the posts after good work from Archie McParland on 65 minutes, and the conversion made it 31-29 for the fifth lead change of the game.
Pearson saw a try chalked off by the TMO for a knock-on from the restart and it nearly came back to haunt Saints as Charlie Atkinson stroked a close-range penalty inside the final four minutes to nudge the visitors into a one-point lead.
Saints threw the kitchen sink at it and Freeman was denied by a fine last-gasp challenge from Ben Redshaw, on his first appearance since March, while a brilliant Thorley tackle denied Fraser Dingwall on the other flank, dragging his boot into touch as he dived to touch down.
There was still time for skipper Furbank to take a short pass and plunge over from close range in the final minute, and though Fin Smith’s conversion attempt was timed-out, Saints held firm from the restart to ensure a home semi-final.