The 39-year-old stepped off the bench as the two sides headed towards a draw after what had been a typical end-of-season clash.
The Poppies' Brett Solkhon struck the post with a header and the ball rebounded to Craig Westcarr. He was driven wide but pulled the ball back and there was his
manager waiting to fire towards goal.
Gainsborough's Ross Greenwood tried desperately to keep it out but it crossed the line to signal the perfect end to a perfect season for Kettering.
There was barely time for Gainsborough to kick off when the referee blew to bring the match to an end.
There had been little sign of what was to come as the two sides were locked together after an even encounter.
The visitors had been the better team in the first half as Kettering struggled to get a grip on things after losing Nicky Eaden and Chris Beardsley to injury inside the first 31 minutes.
Kettering keeper Lee Harper had to be at his best to deny Simon Bird and Ryan Mallon had an effort ruled out for offside as the visitors threatened to complete the only double over the champions.
Westcarr forced Adam Sollitt to tip away a free-kick just before the break and that was the catalyst for the hosts to take control after half-time.
Westcarr saw an effort cleared off the line by Wes Parker after he had robbed Sollitt following a loose backpass from Richard Pell. But the stopper redeemed himself as he pushed away David Bridges' header.
Kettering's pressure finally paid off when Solkhon swept the ball home after Anthony Robinson had created the space for him while John Dempster saw a header cleared off the line by Greenwood soon afterwards.
But Trinity stunned the partisan home crowd with 13 minutes to go when David Reeves converted a penalty after Dempster was adjudged to have handled in the area.
Danny Anson then curled an effort wide. But it seemed the game would end in a draw until the dramatic finale.
Blues manager Steve Charles was rather frustrated with their last-minute winner, saying the referee had signalled three minutes added time but that Kettering's winner had come with four-and-a-half minutes gone.
However, looking back on the season Charles was very pleased with Trinity's final league position of 11th, their highest placing in this grade of football.
"Considering that we had injuries to key players at Christmas and were only a few points above the relegation zone our form since then has been tremendous and I am proud of the effort the players have put in."
He said he had been talking to a number of players to add to the squad but no deals were imminent. He was happy with the nucleus he had retained for next season·