Rajkumar Kaushik daughter's marriage is scheduled for next month, police said.
Around 4pm, six unidentified armed persons, who claimed to be the would-be relatives of Kaushik's daughter, came to his A Block residence in Gagan Vihar area in Sahibabad, they said.
Four armed assailants held the entire family -- Kaushik, his wife Kausum, son Deepak, daughter Natasha and three-year-old grandson -- hostage for 30 minutes, while their two associates were escorting them from outside the house, police said.
The accused robbed Rs 15 lakh in cash, valuables worth Rs three lakh and licensed pistol of property dealer, they said.
After the robbery, all six accused decamped from the spot.
"A manhunt has been launched to nab all six accused," said a senior police officer.