No, while both the Sith and the Grey Jedi are not considered Jedi, that is where their similarities end. The Sith are believers in the power of the Dark Side of the Force and their Code represents their selfish and single-minded beliefs. The Grey Jedi on the other hand, do not think of the Force in terms of Light and Dark. They follow their own beliefs and consider themselves the middle ground between the Jedi and the Sith.