Cody McDonald's first-half winner brought Gillingham their first away win in 18 months with a 1-0 victory at Oxford.
The striker, on a season-long loan from Norwich, bagged his fifth goal of the season in the 40th minute when pouncing on a weak backpass from Oxford's new captain Jake Wright.
McDonald rounded keeper Ryan Clarke and slotted home with ease.
It was a lead Andy Hessenthaler's team, who had not won for 30 away league games, thoroughly deserved.
Charlie Lee also went close with a first-time effort and both Chris Whelpdale and Charlie Lee planted headers wide.
Oxford improved in the second half but slipped to their fifth straight defeat and were booed off by their fans at the end.