Onlookers watched in horror as armed police rushed to the scene of the shooting in Westmuir Street, Parkhead, shortly after 1pm.
The Evening Times understands the victim was targeted while working for City Technical Services in a block of tenement flats.
Witnesses said he was standing in the close, when the gunman burst in and shot him in the shoulder.
Outside the property, a man who is believed to work for the same firm, said: "He's been shot. He's in hospital. It's not serious."
Emergency services were quickly on the scene, including a fleet of police vehicles and an ambulance, which took the victim to hospital.
His condition is not known, but his injuries are not thought to be life-threatening.
The building and a large surrounding area were sealed off.
One neighbour, who did not wish to be named, said: "The guy was working in the close when he got shot.
"I didn't hear a thing, then suddenly there were police with guns all over the street.
"It's getting worse around here."
Police were guarding the main door of the property, as detectives carried out enquiries.
Around two hours after the suspected shooting, white-suited forensic officers
arrived at the scene and could be seen carrying out their investigations.
Officers also carried out a finger-tip search of the pavement outside the tenement.
Locals, including mothers with small children, gathered at the cordon.
As parents collected their children from a nearby school and nursery, shortly after 3pm, some expressed their shock at the