Raymond Anderson and James McDonald had denied murdering Michael Lyons at Applerow Motors in Lambhill.
Both men were also found guilty of attempting to murder Steven Lyons and Robert Picket.
They will be sentenced later at the High Court in Glasgow.
During the trial, the shooting was likened to a scene from The Godfather.
Garage owner David Lyons, the victim's uncle, said two masked men had walked in and began shooting.
The court heard the shooting related to a £25,000 drug debt claim against his nephews.