Former Gillingham favourite Paul Shaw returned to haunt his old club with a goal in each half as Andy Hessenthaler's side slipped deeper into the relegation mire.
Shaw spent over three seasons at Priestfield but left the Kent club less than a year ago for £75,000 after refusing to sign a new contract.
Shaw's brace and Michael Tonge's first goal of the season consigned the relegation-threatened Kent club to their sixth successive home defeat.
Sloppy Gillingham marking allowed United to create a host of chances in the first half and Shaw profited within ten minutes when he volleyed home after a flicked throw-in.
Gillingham's first chance of the game fell to West Ham loanee Adam Nowland whose 35-yard volley was punched over the bar by Paddy Kenny.
Desperate defending from the Gills prevented United taking a two-goal lead when Andrew Crofts headed off the ball off line and two minutes later a last-ditch tackle from Danny Spiller foiled Shaw.
A more purposeful Gillingham side emerged in the second half and Spiller might have grabbed an early equaliser when he connected with a curling Nicky Southall free-kick but he headed the ball straight at Kenny.
Then a deflected cross-shot from Nyron Nosworthy looked to be creeping under the bar but Kenny somehow managed to claw the ball away.
But the game slipped away from Gillingham when suicidal defending by Chris Hope allowed Shaw to bag his second goal with a six-yard tap-in.
Tonge then trebled the Gills' misery when he smashed the ball home from just inside the box after a superb passing move.
Gillingham were offered a glimmer of hope when Chris Morgan received a second yellow card after lashing out at Darius Henderson. The Gills striker had caught him with a loose arm.
With 15 minutes to go United's Leigh Bromby scored Gillingham's first goal at Priestfield since August 14 when he headed a Nowland free-kick into his own net but it was too little too late for the hosts.
