Mrs Goldsmith was knocked to the ground, where she remained with her eyes closed for around 15 seconds before waking up and staggering to her feet, the court heard.
She said: 'The argument took place in the back of a taxi.
Goldsmith pleaded guilty to one count of assault by beating, with prosecutor Kate Shilton describing to the court how Mrs Goldsmith fell down after her husband threw what was described by their taxi driver Daniel Shepherd as a "left hook".
Westminster Magistrates' Court heard that Goldsmith - who is the younger brother of Kate's mother Carole Middleton - left Julie-Ann Goldsmith unconscious outside of their London home.
The court was told Mrs Goldsmith then asked the taxi driver to call the police.
After pleading guilty to one count of assault by beating, Gary, 52, has been released on bail until November 21, when he will be sentenced, reported the Evening Standard.
When at the police station Goldsmith said he had pushed his wife hard but denied hitting her. "She appears to be unconscious, she isn't moving".
Ms Shilton said: "She [Mrs Goldsmith] then becomes upset, starts crying and uses some railings to assist her".
The maximum penalty for his crime is of six months imprisonment, yet he will likely just face community service.
Gary, who is worth nearly $40 million, attended both Kate, 35, and sister Pippa's weddings.
Goldsmith married his first wife Miranda Foote in 1991 and reportedly bought a luxury five-bedroom house in Maidenhead, Berkshire complete with a pool.
She left him amid a reported row over his drug use and he later met and married Julie-Ann.
His father was a painter and decorator from west London and his grandfather worked as a miner in Durham. She met husband Michael at the airline and they later had Kate and Pippa.