Gregor was a freelancing mercenary working for Roberts.
Season 4
Gregor was seen in the basement of a building talking to his friend Jan about which type of bullets they should use for their next job. Roberts entered the basement and introduced Vaughn to the team. Gregor offered Vaughn a drink, but he turned it down at first. Gregor suddenly became hostile and said to either take the drink or "cut bullets with Jan".
Later, Gregor and the team were seen during a briefing held by Vaughn when he declared that their next job, which was to steal an artifact, was to be done his way. The team agreed and proceeded to steal the artifact from an armored truck, which was done successfully. But as they were leaving, Marcus spotted the hostiles and one of Vaughn's teammates (either Gregor or Jan) opened fire, much to Vaughn's dislike. In the ensuing firefight, Gregor was killed by either Marcus or the accompanying APO agent.