Formerly trained by Charlie Hills, the grey was snapped up for 47,000 guineas by David Barron last autumn and looked a shrewd purchase after kicking off this season with successive wins at Thirsk and Newmarket.
He then finished third in the Scurry Handicap at the Curragh before coming home in midfield in the Stewards’ Cup at Goodwood.
The five-year-old was a 10-1 shot for his latest big-race assignment and a quick start allowed Joe Fanning to grab the stands’ rail.
Several challengers tried to bridge the gap on both sides of the track, but Gunmetal was not for catching and came home two and three-quarter lengths clear.
Dakota Gold fared best of those that raced on the far side of the track in second, ahead of Growl in third and Ice Age in fourth.