The court heard the woman had invited Nath to Annabel’s, where she was a member, and they went to the rooftop bar where she ordered the cocktail.
The woman left the table to show two friends around the club, and Clark said two members of staff then saw “rather strange actions” by Nath, who had been left alone with the group’s drinks.
Staff watched as Nath used a straw to take liquid from a bottle he was trying to hide, and then dripped it into the woman’s drink.
“Prior to the police arriving, Mr Nath clearly worked out that he had been rumbled,” Clark said.
“He tried to hide the evidence of what he had done, going to the lavatory and throwing that little bottle into the cistern, and it was floating about when police recovered it.”
In a police interview, Nath said he had got the GBL as a cleaning fluid for a high-performance car, and had been told by a friend that it could also be drunk with alcohol as a “relaxant”.
He told police he had not realised it was illegal for him to be in possession of GBL.
The trial continues.