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.
Gallacher was not the only one who charged into contention. Ulsterman Graeme McDowell turned in 31, and with three to play was six under and joint third with England's Robert Rock.
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 European Tour 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.
McDowell bogeyed the 16th but responded with a tee shot to eight feet on the next and, after holing that, got up and down from sand at the last for his eighth birdie.
He was round in 64 - it would have been a record, of course, but for Gallacher - and the pair were together on seven under, one behind Siem.
Simon Dyson shot 65 to join Gallacher and McDowell on seven under - he lies seventh in the current Ryder Cup points table and victory here could lift him to fifth - and close to securing a debut for himself on the course.
By then, though, Siem was up to nine under after a third birdie of the day on the long 11th and Spain's Gonzalo Fernandez-Castaòo was one behind in second.