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Gillingham's eight-match unbeaten run ended abruptly at Oakwell where they finished well beaten by a resurgent Barnsley.
Two goals in five first half minutes by Chris Lumsdon in the 17th minute and Darren Barnard in the 21st shoved the skids under Gillingham.
A bizarre own goal by Barnsley skipper Chris Morgan on the stroke of half time gave Gillingham hope.
But when Bruce Dyer made it 3-1 in the 48th minute Gillingham dreams of a revival died.
Gary Jones compounded Gillingham's misery by adding a fourth in the 78th minutes.
Attendance: 11,965.