The 34-year-old Yorkshireman beat New Zealand's Paul Coll 3-0 on Monday morning, making it third time lucky after twice losing out to long-time rival Nick Matthew in previous Games.
Willstrop was given a tough time in the first game by his Kiwi opponent, but took control thereafter, closing out the match to win 11-9, 11-4, 11-6.
Afterwards, a delighted Willstrop was hugged by his long-time manager Mick Todd. His victory brought him his first major title after previously having come off second-best in three British Open finals and World Championship final - in addition to his previous disappointments at the Commonwealths, both at the hand of Matthew.
"Things came off, shots went in - it just came together," said Willstrop. "I had to be clinical. He's so good, such an unbelievable athlete. You have to keep faith in the accuracy and keep mixing things. I mixed it well and things just worked.
"I'm not sure what's going on, I can't really believe it!"
Willstrop believed he had saved one of his best-ever performances for one of the biggest games of his career.
"It was the stuff you dream of - most brilliant performances in my career, everything works and clicks," said. "That's happened today. That is one of the best performances.
"There was no expectation. Everything in the lead-up has to go well and it just did. No illnesses for once, it went smoothly, so I knew it was there."
Earlier, Willstrop's England team-mate Sarah-Jane Perry took the silver medal in the women's singles after losing out to New Zealand's Joelle King after a mammoth encounter which saw her defeated 14-16, 8-11, 11-6, 13-12, 8-11.
"I couldn't quite make it over the finish line," said Perry. "Joelly played an absolute blinder at the end. She's a figher and never ever gives up - she's such an amazing player."