Tony McCluskie, 35, claims to have "suffered loss of control" causing the 29-year-old's death on March 1. Prosecutors are still waiting for a medical report to establish how she died.
Tony appeared at the Old Bailey by videolink from Belmarsh prison. His father, Tony senior, watched from the back of the court as he was asked to enter his pleas.
Tony, wearing a dark suit, pleaded not guilty to murder followed by a guilty plea to manslaughter.
Judge Gerald Gordon said this meant a trial on the issue of murder or manslaughter would go ahead on January 14.
An expert on amnesia is expected to be called by the defence.
A trial was due to take place last month but had to be put off because her head was discovered in September.
The actress's headless body was discovered in Regent's Canal on March 6, 2012
The judge said the trial must not be further delayed. He said: "The family must have closure."
Tony lived in Hackney, east London, with his sister. She went missing after attending the opening of the new Royal London Hospital in Whitechapel, east London.
Her limbless body was seen floating in the water a week later. Gemma played Kerry Skinner in the BBC soap in 2001.
