Gregori was a Russian gangster and one of the friends of Iosef Tarasov of the Tarasov Mob. He supported Iosef in the murder of the dog of the titular protagonist, an action who started the chain of events during the series.
Gregori is first seen alongside Iosef and Victor at a gas station where the son of Viggo Tarasov attempt to buy John Wick's car. However, John states that the vehicle isn't for sale, to which Iosef responds in Russian, "Everything's got a price, bitch". Upon hearing John's response (similarly in Russian), "Not this bitch". Iosef seemingly becomes irritated, leaning into the car before being pulled away by Victor, who sarcastically wishes John a good day. Gregori later accompanies Iosef and Victor where they break into John's home and assists Iosef in the killing of Daisy and the thief of the car. He is also present during Iosef's visit at Aurelio's garage and holes the mechanic at gunpoint after he punched Iosef in the face upon hearing that they had not only left John alive.
After John's attempt to kill Iosef at the Red Circle club, Gregori finds refuge with his friend in a safe house in Brooklyn. However, Wick forces Viggo to tell him where Iosef is and uses a sniper rifle taken from one of the lair guards killed to shoot Gregori in the head when he is playing video games, killing him instantly. John then kills Iosef shortly thereafter, exacting revenge for him killing Daisy.