Janemba (ジャネンバ, Janenba) is the main antagonist in the movie Dragon Ball Z: Fusion Reborn and he appears in several other Dragon Ball media. He is a demon of pure evil and great power. Janemba is the living definition of evil.
It's not Teen Gohan, but one of his descendants, after Goku Jr. from GT. His name is Barku, named after Bardock and Goku. He achieved his transformation while fighting a Saiyan from Universe for whom cut off his hand.