Halfpenny's second set up the chase for a bonus point and it came with seven minutes to go when Steff Evans flew over down the left.
Scotland replacement Peter Horne went over late on and Finn Russell converted, but it was little consolation as Wales claimed their 50th win of Gatland's reign.
Barclay, who has assumed the captaincy from Greig Laidlaw, said at his eve-of-match press conference that he doubted Gatland would change a formula of play that has served him well in 10 years as Wales coach.
In the opening five minutes of this afternoon's clash between Wales and Scotland at the Principality Stadium, it looked like Gregor Townsend's side had come to play.
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Mattis told reporters the Syrian government had committed multiple attacks using chlorine gas. He added "they'd be ill-advised to go back to violating the chemical convention".
Scotland settled and drew the majority of possession but kept dropping passes or conceding penalties and didn't stress the Wales defense until the very end.Wales fullback Leigh Halfpenny, who hadn't scored a test try in five years, scored two for the first time since the same fixture in 2012.
"That was a massive performance", said the back-row.
"We went to Murrayfield previous year with a view of being dominant, which we were for large parts of the game, but a couple of scrums went down, we brought the referee into it, and then some 50-50 calls went against us".
Wales: L Halfpenny; J Adams, S Williams (O Watkin 70), H Parkes, S Evans; R Patchell (G Anscombe 63), G Davies (A Davies 65); R Evans (W Jones 50), K Owens (E Dee 63), S Lee (T Francis 50), C Hill (B Davies 55), A Wyn Jones, A Shingler, J Navidi, R Moriarty (J Tipuric 64). But his offload wasn't as good as his forwards' support.
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John Barclay was the transgressor on that occasion and the visiting captain was penalised again shortly afterwards, Halfpenny thumping another kick straight down the middle from just inside the Scotland half.
Scotland dominated the near half-hour to halftime but lapsed into error after error: Dropped passes, knock ons, a penalty kick not finding touch, crooked throw-ins, and the backs shuffling sideways to be easily contained by Wales. The conversion made it a personal tally of 22 for Halfpenny in Wales' 27-0 lead.
"They've got the best if not one of the best stadiums in world rugby and it's unbelievable to play down there, so they must get a buzz off it".
The Scarlets prop then passed back inside to Alun Wyn Jones only for Steffan Evans to spill the ball forward with the line at his mercy.
Scotland ladies will deploy all their four full-time experts in Friday's opening match of their Six Nations crusade against Wales.
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Joe Manchin , one of the most politically vulnerable Democrats in the country. Every ounce of me said: "Get home to Texas as fast as you can", said Rep.