Table-toppers Gills slipped to their sixth loss of the season at the Priestfield Stadium after Alfie Potter's 84th-minute strike gave mid-table Oxford a surprise win.
The Valiants could have leapfrogged Martin Allen's men, but they succumbed to defeat to 10-man Exeter.
Jamie Cureton and Danny Coles gave the visitors a two-goal lead before the break and although Mark Molesley was dismissed for a second bookable offence, the Grecians comfortably held on.
That has helped Burton narrow the gap to four points on the Valiants after they held off a resilient Morecambe to triumph 3-2.
The Brewers looked home and hosed after Jacques Maghoma's opener was added to by Billy Kee's brace although Jack Redshaw's penalty and Chris Holroyd's strike late on kept the hosts sweating.
Northampton are breathing down their necks - just a point and a place behind Albion - after Ben Harding's 88th-minute effort helped the Cobblers edge out Bristol Rovers 1-0.
Rotherham slipped three points behind Burton, but were grateful for Daniel Nardiello's added-time goal in their 1-1 draw at home to York.
The Minstermen appeared on course for a first win in 10 matches when Matty Blair gave them the lead just after the hour, but Nardiello's strike in the third minute of injury-time ensured the spoils were shared.
Three goals in the final 14 minutes helped Accrington move out of the relegation zone after a 3-0 win at Rochdale.
Lee Molyneux scored in the 76th and 80th minutes before Romuald Boco sealed an ultimately comfortable win four minutes from time.
AFC Wimbledon now occupy the final relegation spot, despite Harry Pell's 28th-minute leveller helping his side to a 1-1 draw at home to rock-bottom Plymouth, who had taken the lead two minutes earlier through Jason Banton.
Barnet remain one point above the drop zone, despite Edgar Davids inspiring them to a 2-0 victory over Southend.
The Bees player-manager claimed his first goal for the club in the 84th minute after Jake Hyde had put them ahead just after the half hour.
Sam Wood and Matt McClure were on-target in the second half as Wycombe hit back to claim a 2-1 win at Torquay, who had taken the lead through Ryan Jarvis in the 37th minute.
Junior Brown cancelled out Marlon Pack's opener as Fleetwood drew 1-1 with Cheltenham, while Chesterfield were held to a goalless draw by Aldershot.