The most important:
1. The film shows Castellano forcing Gotti to take Ralph (Ralphie Wigs) Galione on the McBratney hit. In real life, Gotti and Ruggiero chose Galione.
2. Film shows Galione coked up and trigger-happily shooting McBratney. In reality, McBratney resisted all three very strongly, and had to be shot.
3. Film shows Gotti whacking Galione for screwing up the McBratney hit, and Castellano demanding Gotti's death as a result. In reality, Galione was killed almost two years later in a dispute with a loan shark that had nothing to do with the McBratney hit. And Castellano didn't demand Gotti's death.
4. Film shows Da Bull organizing and pulling the trigger in the murder, in an underpass, of John Favara, Gotti's unfortunate neighbor who accidentally ran over his son. In reality, Gravano probably had nothing to do with the Favara murder. And he was clubbed unconscious in the parking lot of a diner, stuffed into a van and never seen again.