The Gulls defence was caught out by a swift build-up and their young keeper, Martin Horsell, was stranded at the wrong post as Craig Disley hit in a long cross from the right.
Playing only his second league match, the 20-year-old keeper had kept his side in with a chance of a point with a series of fine saves.
He was praised by his Czech manager Lubos Kubik who still has to see his side win in the league since he took over in November.
United have now gone 17 games without a win but it took Rovers until the last 20 minutes to get on top.
A heavy storm which swept the ground in the first half left behind treacherous pools of water and sticky conditions.
In the first half it was Torquay who made the running with Lee Thorpe and young Jamie Ward stretching Rovers' defence.
With Chris McPhee breaking fast from the midfield, Torquay won eight first-half corners without really finding a shot to worry Steve Phillips.
Thorpe, relishing a game against one of this old clubs, was a lively attacker and set up Torquay's best move. He fell deep to come away with the ball and sent Ward away on the right.
The move won two corners but Ward had to wait until the start of the second half to get in his first shot.
Then, turning on the edge of the box, he fired at Phillips but the pace of his shot was taken off by the mud and the keeper had little problems saving the effort.
Ward, who allowed his frustrations to lead to a booking, was much closer later when Phillips had to make a point-blank save to deny him.
Rovers replaced Lewis Haldane at half-time with Sandell and it proved the right move as the substitute began to get the better of Lee Andrews, before eventually grabbing the winner.