Gills, five points clear at the start of play, could do no better than a goalless draw at Oxford, while Vale managed a 2-0 win.
Tom Pope was their match-winner, notching in the 40th and 62nd minutes.
Fleetwood remained in third spot after salvaging a 2-2 draw at Cheltenham.
David Ball gave the visitors the lead after 32 minutes but Jeff Goulding and Sido Jombati scored in 10 second-half minutes to turn the score on its head. However, Junior Brown saved a point for Fleetwood in injury time before Alan Bennett was handed a late red card.
Rochdale leapt from ninth to fifth with a 3-2 win at Accrington, Jason Kennedy with the winner seven minutes from time.
George Donnelly scored in the first minute for Dale and Ashley Grimes added a second from the penalty spot but Piero Mingoia and Romuald Boco netted either side of half-time in reply.
Bottom side Barnet picked up just their third point of the season with a 2-2 draw at Southend.
Goals from Rickie Holmes (54) and Jake Hyde (65) had raised hopes of an even better result but a Freddy Eastwood penalty and a Kevan Hurst effort with six minutes left put paid to that.
Dagenham edged an entertaining clash with visiting Bradford 4-3.
Billy Bingham, Michael Spillane and Luke Wilkinson gave the home team a three-goal cushion before a Nakhi Wells brace gave the Bantams hope.
Dwight Gayle grabbed the Daggers' fourth before Alan Connell rounded out the scoring for Bradford.
Chesterfield were 1-0 victors over Aldershot, thanks to Chris Atkinson's goal in the 76th minute.
Northampton lost Anthony Charles to a 24th-minute red card and could not overcome the disadvantage as they went down 3-1 at Bristol Rovers.
Tom Eaves (53), Garry Kenneth (57) and Oliver Norburn (84) all took advantage to score, with Adebayo Akinfenwa on hand with a consolation.
AFC Wimbledon turned one point into three when Rashid Yussuff snatched a last-gasp winner in a 2-1 success over Plymouth.
Byron Harrison's 13th-minute strike looked to be enough for the visitors only for Argyle to hit back with just a minute to go via Rhys Griffiths, though Yussuff still had time to win it.
Wycombe defeated Torquay 2-1 after goals from Gary Doherty and Josh Scowen negated Rene Howe's effort.
York versus Rotherham and Morecambe against Burton Albion both finished goalless.