• Gallacher broke the course record with a round of 63 Pic:Getty
Seven off the lead at the start of the day, the nephew of former Ryder Cup captain Bernard followed six birdies with a brilliant closing eagle, hitting a 242-yard rescue club to within three feet of the flag.
The performance lifted him all the way into a share of the lead with Marcel Siem, but the German then inched ahead again with a second birdie of the day on the 433-yard fifth. Siem eventually finished on 66 to take a three-shot lead from Denmark's Thomas Bjorn and Gonzalo Fdez-Castano of Spain. Gallacher is one shot further back in a share of fourth.
All this came while Gallacher's compatriot Colin Montgomerie was having an absolute shocker the day after newspaper revelations about his private life.
Europe's current Ryder Cup captain – this is the course on which America will defend the trophy in October – resumed only four behind Siem and joint overnight leader Andrew Dodt.
But Montgomerie, who says he and wife Gaynor are "trying to resolve the issues" after he was linked with a former girlfriend, turned in 42 and dropped another stroke at the 10th to slump 13 shots adrift and out of the running for a first win in three years.
The 46-year-old three-putted the long second for a six, went in the water to drop two more on the 189-yard next, bogeyed the fifth and then ran up a double-bogey seven at the ninth.
Gallacher was not the only one who charged into contention. Ulsterman Graeme McDowell turned in 31, 11 fewer than Montgomerie, and finished with 64 to join Gallacher on 206.
This time last year 35-year-old Gallacher was fighting the viral infection sarcoidosis and even had a biopsy to test for cancer. Steroid treatment brought a cure, but he confesses he has felt "absolutely shattered" this week. "I've been knackered, but I've just got a wee bit of belief in myself," he said.
The former Dunhill Links champion returned to the qualifying school last November, but has no such worries this season after finishing fourth at the BMW PGA Championship two weeks ago. "Wentworth was big, that took the pressure off the card for next year and meant I could enjoy it more," he added.
Montgomerie, meanwhile, finished with a 76 to be three over, but amid the disappointment of falling out of the title picture himself it had not escaped him that cup contender Simon Dyson, playing just ahead of him, shot 65 to join Gallacher and McDowell on seven under.
Dyson, congratulated by Montgomerie as he was being interviewed, lies seventh in the current points table and victory today could lift him to fifth – and close to securing a debut for himself on the course.
"He watched me all day and I didn't miss a shot," said the York golfer. "I knew he was there and you want to show what you can do. It's amazing how you up your game.
"I've not even looked at the cup table and don't know where I am, so don't tell me," added the 32-year-old. "Win tomorrow and I'll have a look maybe."