He raced on to an underhit backpass from Andy Hessenthaler to place a cool shot across Barnet keeper Lee Harrison's body and just out of reach of his left hand.
Within minutes flustered keeper Harrison almost handed Rovers a second goal when he came off his line to catch another backpass.
This time it didn't work out quite so well for Nicholson for he was booked trying to speed up the free-kick by wrestling the ball out of Harrison's hands.
It all sparked a minor push and shove in the penalty area and when things quietened down Stuart Campbell, who has now seen Rovers win five in a row since taking over the captaincy, fired in a shot which went behind off the wall and a post.
Nicholson a self-confessed Rovers "nut" wrote in for a trial with them four years ago - he never got a reply - before signing on at The Hawthorns.
He was the difference between two sides, who struggled on a pitch roughed up by a rainy Friday night rugby match between Bristol and Gloucester and it took a rugby-type run by Rickie Lambert, handing off two challenges to wrap up the win in added-on time.
Barnet have yet to beat Rovers, but in the first half were the more assertive side with Ismail Yakubu solid in defence and Dean Sinclair a lively raider.
It was Sinclair who so nearly gave Barnet the lead in the first half when he bounced a long hard drive off home keeper Steve Phillips.
Harrison always ready to come off his line, rather bizarrely raced 50 yards to protest to the referee after just seven minutes when Bees skipper Ian Hendon, back after missing six games with a thigh injury, was rather mysteriously booked on the far touchline.
Harrison escaped without a rebuke, but had a far luckier escape just before the break when Nicholson might have beaten him.
The on-loan striker had raced away from the halfway line rounded Harrison and was sliding the ball home, before being denied by Simon King who hurled himself at the ball and made a sliding clearance.