The father-son duo, Satveer and Joginger, were seriously injured in the attack yesterday, allegedly over a property related dispute.
They were later shifted to Medanta Medicity hospital after the attack inside Sunrise hospital yesterday.
Joginder succumbed to his injuries this morning while his father, who has now been shifted to a government hospital in Delhi, remains to be in critical condition, hospital sources said.
Meanwhile, Gurgaon police have arrested the main accused Manoj alias Aalu.
According to Deputy Commissioner of Police (Crime), Gurgaon, Maheshwer Dayal, Manoj had fired shots at the duo inside Sunrise hospital yesterday.
"Manoj was arrested from Gurgaon last night when he was trying to escape while his other accomplices managed to flee," Dayal said.
The police have identified seven accused and would soon arrest rest of them, the officer claimed.
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Last evening, over seven persons had attacked the father-son duo in the Intensive Care Unit (ICU) of the private hospital in the satellite town.
"Both the parties had fought outside Gurgaon court yesterday after a hearing, and the father-son duo were admitted to the hospital after receiving minor injuries," the DCP said.
According to eyewitnesses, the men, numbered around seven-eight, barged into the Sunrise hospital at around 5 PM and shot at the duo.
A guard at the hospital said, "the men were armed and I fled soon after hearing five gun shots."