The Gnostics associated the Old Testament Jehovah with the Platonic concept of the Demiurge—an imperfect or misguided lesser deity who created the material world but botched it up and included evil as an unintended consequence—as opposed to the New Testament “God” who was the Platonic principle of transcendent Goodness or Unity. So the Gnostics didn’t need a separate Satan, since Jehovah was already covering that role.