The Gills only needed a point to seal the League Two championship and their 2-2 draw with AFC Wimbledon did just that, with the Kent club establishing a comfortable advantage inside 23 minutes thanks to Deon Burton's neat volley and a header from former Dons striker Danny Kedwell.
However, Jack Midson halved the deficit with 25 minutes remaining and Jonathan Meades slotted home a late equaliser for the relegation-threatened Dons.
But despite seeing his side squander a two-goal lead, it was enough to pop the champagne corks and delight manager Martin Allen.
"It's been an amazing 10 months," he said. "The players have kept going all the way through the season, and to be the manager on this special day is the highlight of my career.
"I've been known as a manager who keeps clubs up, so to be champions is unbelievable. It's a lottery win, a Christmas Day, a priceless moment."
For AFC Wimbledon it is the other end of the table that is causing concern with Neal Ardley's side beginning the final day in the drop zone.
"I thought there was only going to be one winner at the end," Ardley said. "All we needed was one ball dropping to the right player at the right time, but ultimately the players gave it all they could.
"I'm really disappointed with the first half, we worked so hard all week looking at how we were going to go about the game, and then we did completely the opposite.
"If we can play like we did after the break next week, we can stay up. I still believe that even though we've only picked up a couple of points in the last few games, we've not been playing badly."