Danny Kedwell's 15th goal of the season, a fine volley from Joe Martin's diagonal pass, sealed victory for the Gills over strugglers Torquay and with Northampton and Rotherham both losing, promotion was confirmed.
Only Port Vale can catch Allen's side - unbeaten in their last nine - and Gillingham could be crowned champions with a win at Cheltenham next week.
And Allen is adamant his side will prove they can cut it in League One in the remaining weeks of the season.
"It's an amazing feeling," he said. "It's been a team effort on and off the field, a lot of people have played their part and helped this club.
"We've got proper men in the team, and we've got some young lads who are full of energy and enthusiasm. I had total faith in everybody that they would do the job.
"We're work in progress, that's for sure. There's a lot of work left to be done and we're all looking forward to going out to win the title. First and foremost, our challenge was to get promoted.
"But now we've got the chance to win the championship. We decided a few weeks that we would aim to do that. We could have chucked the kids in, and had a bit of a laugh until the end of the season.
"The players don't want that and their drive over the past weeks has been about going the whole way."
- Sports & Recreation