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.”
At least four people have been killed, including three Chinese nationals.
Three Chinese language teachers and their Pakistani driver were killed in a suspected suicide bombing in the southern city of Karachi, police say.
The blast ripped through their minibus, injuring at least four others near the university's Confucius Institute.
The separatist Baloch Liberation Army (BLA) said it attacked the vehicle carrying the Chinese staff, and that the suicide bomber had been a woman.
The group opposes Chinese investment in Pakistan, saying locals do not benefit.
It would be the first time a suicide attack by the BLA has been carried out by a female militant. The group has targeted Chinese nationals on a number of occasions, as has the Pakistani Taliban.
China is heavily involved in large infrastructure projects across Pakistan, including in resource-rich Balochistan province.
China's embassy in Pakistan confirmed that three of the country's citizens had been killed in Tuesday's blast. Pakistan's government condemned what it called a "cowardly terrorist attack".
The dead included the director of the Confucius Institute, a Chinese government-run body which offers language and cultural programmes overseas, and two other faculty members in Karachi.
CCTV footage showed a woman standing outside the gate of the institute as the van arrives, followed by a powerful explosion.