Gregg appeared in the 1989 hit movie Indiana Jones and the Last Crusade (another film with River Phoenix) and had the lead role in the 1990 action/science-fiction movie Class of 1999. His most recent film was Whiplash (2002). He starred on the short-lived TV series The Marshall Chronicles and starred in the 1992 TV movie O Pioneers! with Jessica Lange.
Gregg made his directorial debut in the movie 14 Days in America for which he was also producer and editor. He took five years off from acting until 2005. He has been living in the desert of Arizona directing, editing, and writing films with his company "Eventide Fields".