I read that the part about the psychic was almosst completely fabricated. While they did ask for help from detective psychic Dorothy Allison, she did not contribute to solving the case. In fact they even drove around with her for hours, searching for the body to no avail.
Not a fabrication. There were 2 psychics. Carol something or other was the first one told them of tremendous pain and multiple bodies (which they did not know they were looking for at the time). She did not have the location of any of the bodies correct but she did help in some way.
The second psychic was Dorothy Allison and allegedly it was at the request of Mrs. Piest who wanted a proper burial for Robert, was contacted to help locate the body. She had the location correct.
Read "Killer Clown" by Terry Sullivan (prosecutor) and/or "Defending a Monster" by Sam Amirante (Defense) for more accurate information on the case. Both say basically the same things about how the case went with a different point of view.
Gacy's full transcribed confession is in Defending A Monster.
What about when Margo's character mentioned the killer in Texas that she hoped Gacy wasn't like? She said his name was Arthur Hanley and that he murdered 12 boys. Was there really such a guy out there before Gacy's final arrest? So far, I can't find anything on such a guy on the internet.
The movie's disclaimer said that some names were changed for the movie. Could that mean names of places as well as people? Why would they care about the real name of a confirmed killer?
Edit: It might be Dean Coril, who died in Texas of gun shot in 1973. It was known as the worst case of serial murder in its time. One of his accomplices was named Elmer Henley, which could sound like Arthur Hanley. But Dean was the head guy.
Any other possibilities? Could they have changed the name of the state to Texas?