Ryder accused the Braveheart star of levelling anti-Semitic remarks at her, and homophobic slurs at her friend, during a 1995 Hollywood party.
The Oscar-winning director's representatives denied Ryder's allegations in an emailed statement to the Post.
"This is 100 per cent untrue," they said, also claiming that the Stranger Things star lied in the 2010 GQ article in which she first mentioned the alleged incident.
But Ryder stood by her claims: "I believe in redemption and forgiveness and hope that Mr Gibson has found a healthy way to deal with his demons, but I am not one of them," she said.
Gibson's coronavirus revelation makes him the latest in a string of celebrities to have been hit with the virus: Tom Hanks and wife Rita Wilson were two of the earliest A-list patients, coming down with Covid-19 during an Australian visit in March.
Broadway actor Nick Cordero's brutal coronavirus battle came to an end this month after more than three months in hospital: after losing a leg, being fitted with a pacemaker and suffering a lung infection, he died, aged 41.
And Pitch Perfect star Anna Camp revealed this week that she's been diagnosed with coronavirus, despite being very careful: "One time, when the world was starting to open up, I decided to forgo wearing my mask in public. One. Time. And I ended up getting it," she wrote.
