The expat, from Hastings, East Sussex, was working out at the Coco Fitness gym when he was confronted by another Englishman.
The 46-year-old says the furious fitness fanatic complained that he had been waiting a while to use the bench press - despite there being numerous others available.
After explaining he wanted to finish his set, Gary assumed the man had shrugged off his impatience and was willing to wait.
He explained: "I was like 'mate, just let me finish, I'll be done in two minutes', but he wasn't having it.
"There were others available to use, so I think he was looking for a fight."
But just moments later, the brute charged over to the Brit with a seven-inch long 1kg metal dumbbell bar and horrifically smacked him in the face from behind.
The sickening surveillance footage shows the attacker, dressed in blue, striking an unsuspecting Gary as he continued his workout.
The Sussex-born fitness lover said: "He just came at me from behind like a coward and proper smashed me in the head.”
WYOMING, Mich. (AP) — A rowdy visitor at a Grand Rapids-area hotel set up a phone to try to record the fatal shooting of a security guard who was sent to remove him, police said.
The 21-year-old gunman then went to his mother's house nearby and killed himself, police said.
The incident occurred Tuesday at the Grand Rapids Inn in Wyoming. The security guard was identified as John Cain Jr.
Cain, 38, was “just trying to protect their property, do what they wanted done for removing people that they didn’t want at the hotel,” Lt. Brian Look said, referring to management.
The 21-year-old gunman was identified as Jacob Grygorzyk. His video was posted on social media but subsequently removed.
It “didn’t actually show the shooting,” Look said. “You could hear that in the background.”
John Cain Sr. said the gunman “took the easy way out.”
“My son, who’s got my name, who I raised, is no longer here," Cain told MLive.com