United opened the scoring on 16 minutes with the first real threat of the match and it was their centre-half partnership which caused the problems.
James O'Brien's corner was met by the head of Darren Dods, his effort was deflected onto the crossbar and the rebound fell kindly for Garry Kenneth, who stuck out a leg to force the ball into the net.
The visitors continued to push forward and they could have made it two just before the half-hour mark. Christian Kalvenes sent in a low cross to the near post, but David Robertson could only turn the ball over at full stretch.
Apart from a Noel Hunt snapshot which was saved low to his right by Greg Fleming these were the only highlights in a largely disappointing first half, particularly given how important points were meant to be for both sides.
Gretna almost equalised five minutes after the break, but Lukasz Zaluska made a fantastic stop to keep out Gavin Skelton's low drive. Within a minute the home side found themselves two goals down.
Hunt broke down the right and although his cross was initially missed by O'Brien, it reached Morgaro Gomis, who fired low past Fleming.
Gretna plugged away and Brendan McGill almost cashed in on a mad dash off his line by United keeper Zaluska, but his angled drive bounced clear off the post.
Yet again the home side lost a goal within minutes, Hunt appeared to have made a mess of a long ball, but the ball eventually sat nicely for Kalvenes.
The full-back's cross found Robertson in acres of space to sidefoot past the helpless Fleming.
This effectively sealed the points and allowed United to coast to victory without any further undue alarm.