Conversation so far:
What was Vidal's first screenplay?
In 1958, the director William Wyler required a script doctor to rewrite the screenplay for Ben-Hur (1959).
How did Vidal become involved in Ben-Hur?
Vidal rewrote portions of the script to resolve ambiguities of character motivation.
Was his work on Ben-Hur successful for him?
In exchange for rewriting the Ben-Hur screenplay, on location in Italy, Vidal negotiated the early termination (at the two-year mark) of his four-year contract with MGM.
Where did he go after that?
Vidal occasionally returned to the movie business, and wrote historically accurate teleplays and screenplays about subjects important to him.
Were any of those screenplays notable?
Two such movies are the cowboy movie Billy the Kid (1989).
and the other movie?
the Roman Empire movie Caligula (1979), from which Vidal had his screenwriter credit removed.

Question: why did he have his credit removed?
Self-contained rewrite: Why did Gore Vidal have his credit removed?