Gregori's only known family is his son, Yuri Leonov.
The Overlord Protocol
Gregori Leonov briefly appeared at the start of The Overlord Protocol, where he was one of the global leaders of villainy at the meeting in Vienna. He discussed his son's recent successes with Nero.
Gregori also appeared in Escape Velocity, when he met with Nero in London. The pair travelled on the London Eye, but Leonov was shot and killed by H.O.P.E. forces.